Its basically impossible to camp a spawn (unless its your own lol).
Someone may get lucky and have a person spawn right in front of them on occasion but as far as camping a spawn area and getting more than a few kills a match off of it would be rare.
In general if you camp an area, the enemy will spawn away from that.
But if you are by some chance having trouble with sentry guns in general, have you a blind eye class set up. Or you could judt not run back in front of the sentry gun.
Also, as mentioned above sentry guns are easier to destroy than ever with EMP's. EMP's make Sentry guns almost worthless.
