So some people farm.  Who cares?  Honestly.  Why spend that much time and effort banning people that choose to do things that hardly affect others.  Banning from your own server is alright if you want to hold legitimate games and people are farming crap and avoiding the game objectives, but devising a method to ban people from multiple server simultaneously is more pathetic than farming could ever be.

I'd be more concerned about people that bypass achievements entirely with the unlockers.

That being said, I can understand the dislike of people going to heavily modded servers with bots and getting the achievements that way, but I don't understand why people think farming is so bad otherwise on legitimate servers.  I had a blast farming achievements with some of my old clan members.  It was fun as hell: we all got together on vent for the first time in ages and did every silly step together to get the achievements and we had a blast doing it.  One of my friends thought he was the one that had to jump when he got killed for "The Cycle" and we kept wondering wtf he was doing, which of course was funny as hell.  People that think farming is a cheap need to open their mind a little bit.  I had more fun farming with old friends than I had playing the regular game in months.  By the way, Valve has no problem with farming.