With White Scars being everyone's favorite bandwagon to jump on currently, let's look at Scout protection and why you might need this.
Scouts for individual units is great at helping them put pressure on an opponent or get them into a slightly better position. Scouts for entire armies magnifies the pressure they can put on an opponent – particularly if they are fast. White Scars are obviously fast and combined with natural Hit and Run – this means they can effectively take away entire shooting phases from large parts of your army Turn 2 onwards. It's therefore damn important to know how to stop or restrict this – they are a fast army so they're likely to get into combat with you at some point or running shooting circles around you but you want dictate how they do that as much as physically possible.

The first and most obvious answer here is using infiltrators and bubble-wraps. Bubble-wraps are no longer as powerful as they were in 5th edition however; they can still limit the engagement opportunities for opponents using an aggressive Scout based list (remember, not just White Scars have this option but so do Raven Guard, Khorne Dogs, Grey Knights, etc.) which puts you back into the controlling seat of what's unfolding on the battlefield. You can't always stop an opponent from doing something but if they're doing it the way you want, it's a lot easier to win a game. Then there's infiltrating units – one of the core restrictions of Scouting is you cannot end within 12″ of an opposing unit. This is obviously a great way to push Scouting units back by simply extending the presence of your army outside of your own deployment zone. Although the bigger the unit here, the more area you can keep Scouting units away from, even a token Infiltrator can assist in giving you that extra turn of space.

Taking this further and we combine the two concepts – an infiltrating bubble-wrap. Kroot are the most obvious example here where not only are they acting as a bubble-wrap but by infiltrating, they are providing Tau with more space to utilise their own movement and tactical options to win the game. Not only is the unit protecting your army but it's also keeping Scouting units away from the area you wish to control and its often part of the army to begin with anyway. Given the rise in these type of armies recently, I'd be looking very hard at having some sort of infiltrating unit – it doesn't have to be big or expensive but expanding the range of your ability to push Scouts away from this:

Scout1
 
to this:

Scout2

is massive. It might not seem like much (6″ linear) but it takes a lot more of the board away from your opponent – particularly if you have a bigger unit or multiple small uits to create larger board coverage. If you can infiltrate out of LoS as well, they effectively cannot get closer to you. That's massive x2. That being said, you want to make sure the unit can otherwise add something to your army and isn't a huge useless lobbie otherwise (oh hi Mandrakes!) but given the strong push for these armies currently, that change is game-changing. Remember as well, this concept can be used against podding or deepstriking armies though the aura effect is replaced by simple bodies in the way.

Be very careful if your opponent has their own Infiltrators however; as this means they can effectively push back your infiltrators and give their Scout units the free reign they wish. That being said – many Scouting armies won't bring something like this as it's only a 50/50 chance for it to work and it's not always going to be worth the points investment given that many players won't bring anything with this rule – I imagine this will increase though and for often the paltry cost of 75 points (i.e. Sniper Scouts), armies like this are going to be advantaged by bringing them. The same goes for opposing Scouting units – it can push forth the bubble which they cannot move into however; your opponent has an opportunity to win a 50/50 roll-off here which means you're not always going to get what you want so it's not the most reliable defense.
There are other methods to eliminate and reduce Scouting threats for some armies such as Servo Skulls. Although these wargear pieces are not common, Allies opens this up massively to anyone who may or may not be struggling with Scouts (and off the top of my head, I cannot think of other anti-Scouting wargear pieces currently…).

But what happens if you cannot access such wargear or don't have your own Scouts/Infiltrators? Well first – consider your army list and see if this is a change which can be made which isn't going to negatively impact its ability to wage tiny D6 war. If not – Scouts offers movement options to the opponent before the game starts which either involves getting into better firing positions or aggressive positions. The above options look to take that away outside of your normal deployment but it doesn't restrict opponents from actually getting benefits – they still can; it's just you're attempting to control some of that yourself. Regardless, you want to try and minimise what they are aiming to do. If they're trying to get Gargoyles, Hounds, Bikes, whatever into your main army as fast as possible to stop them shooting and bog them in place, put your main army somewhere where they are difficult to get to. Terrain, bubble-wrap units, impassable objects, open table between units, etc.

Use your own Hit & Run, have super durable tarpits in places you don't want to actually go, etc. I so often see people setting up their defenses on objectives or choke-points between buildings – your defenses are past these points so your army can move to hold these as they need. You need to afford yourself the movement options behind what will inevitably be attacked or shot so you are giving yourself tactical options to respond to your opponent. When push comes to shove – you cannot take away all the advantages of Scout – even if you have enough Servo Skulls to stop all Scouting – Scouts can still redeploy to overcome a refused flank, reposition weapons better, rearrange units, etc. but the more you try and fight on your own terms, the less advantage your opponent may be able to gain from this.