just run away and LoS them
1) Find a pole, or corner, or anything that blocks LOS
2) Stand near it
3) Use instant cast ranged attacks on sniper
4) When you see his cast bar start to move, step behind cover
5) His casting bar says "Failed" because you LOS
6) Step out and attack again
7) Go to step 4
You will test their patience since they wont be able to hit you with anything that's channeled.
See here is the thing, If you los we can just kill someone else, I have had folks los me as the ball carrier goes right by them or a team mate walked up and took the node all cause they are worried about me. So ya i didnt get to kill them but I did my job. When played against right sniper gunslingers dont live long. I always go in with the thinking that if i can piss off enough folks then they will be worrying about me and not the game we are playing. I dont mind, especially if I can take someone with me to a respawn point hehe.