You know what gives an advantage? Hiding in said bush/haystack/whatever with a crossbow,seeing above/through it (either due to 3rd person camera or because of how haystacks work(you can see through them , if your camera is inside)), whilst not being visible to others. Tell me, Tavuk_Bey, are you a crossbowman? Do you, by any chance, do this?
Also stop assuming you have such support in this.
Hiding in haystack is not against the rules (unless you are delaying at the end of the round). Its not relevant if you think its "fair" or not, if you think not then try to get the rules changed. Removing haystacks/bushes/rocks/walls etc is most likely against an unwritten rule but it should be more clear. Otherwise we might as well all remove those parts of the game. I would, I believe in people playing on an even playing field even if that means we strip down the game to a "normal" game setting that includes no hay,bushes etc
When it comes to punishment, I really don't see how you can tell who cheats without treating them like you do wallhackers in other games. You have to demo/fraps them for hours to be completely sure they cheat, and even then there may be some doubt if the player isn't completely full retard about hiding his hacks. You will also get players that are good at hiding their ability to see players, and will be more subtle about it and hard to catch.
When you do catch them though after all that effort it should be a real perma ban. If people know you have to go to that trouble to get someone banned then people will do it until they get their first warning