Over faming is when u farm the same Area so much that Anets anti farming code kicks in which reduces all ur drop rates until u go do other things to let it raise back up again. no this on its own cant get u into trouble with anet, but farming to sell for RL money and using bots to farm for u is.
If u farm one area too much youll trigger anti-farming code that will reduce the drops. Thats all that can happen to you if you farm a lot. If you change farming areas frequently and do some missions from time to time you should be OK.
Yes, it can get you in trouble with Anet. If anti-farm ever trigger on you, you'll be on Anet's blacklist, where they'll keep a close watch on you. But no...they won't ban you until you do something that violate the rule.