You cannot really blame them for using their brain when they play. It is a solid tactic.
The best way to counter something like this is teamwork. Actual team players are hard to find on the internet unless you have a group of friends who all play together all the time. I will use CoD which is what I play most of the time to better explain what I mean. (I was a former MLG compe or so I have pretty decent strategy I suppose.)
Most MMOFPS have the same basic principles concerning spawn points, and spawn switches. In CoD, when a certain percentage of your team crosses the halfway point in the map the spawn point will change to negate spawn camping. What we would do is first make sure EVERYONE on our team understood this. This is something that you will need to have the people you play with understand because it prevents the enemy from spawning behind your positions. This strategy basically is what everyone should use. We would use snipers very well by taking out the enemy before they could set up a position. This would give us the lead. Our defense would then be set and it would force the enemy to come to us in order to win the match. With all of our players in predesignated positions to offer maximum firepower and flank protection, the slaughter began because all the entry points were covered by normally 2 different reticules.
If those ers keep using a smart tactic like that, get the lead using speed and crack distance shooting/grenades, while positioning your team. They won't be able to use it anymore because in order to win, or increase their K

ratio or KPM, they have to come to you.
If you don't have the lead, don't move out on your own, always have someone covering your movement (prefereably a sniper or someone with a long range weapon with 1 shot kill capability) to potentially lure an enemy out of cover. If 2 enemies appear, the ranged guy could possibly get 2 kills for 1 lost teammate. If it is just one, it could even out and the location of the defender is given away. If he is killed or moved out of position, don't take his position, instead just guard the entrances to get there. Campers/turtlers are notorious for wanting to get to their "sweet spot". You should get a few kills depending on how stubborn he is. Then once the lead is gained, fall back to your predesignated position with your team.
Don't only think of a battlefield as just eliminating your opponents like a chess game. You also have to be able to control the battlefield by being able to occupy the most space without stretching yourself too thin. Kinda like the game Go.