Seeing as how everything is kept online anyways, you would instants have statistical data on every character - what skills they used, what area they hit, and how often they were in a particular area in any given time.
Human players are adaptable. Bots are not.
The game environment should mimic the real world somewhat. The creatures in it should be able to learn and adapt to repeated 'attacks' by the same tactics. It's a war in the game, it should be treated as a war would be in the real world - tactics adapting to combat tactics.
Ok, that's the lead-in...
Your average bot is going to do the same thing, the same way, over and over again. They find a nice farmable spot with a certain creature criteria - usually melee-only creatures is the easiest - and then hit the area over and over and over. I don't care if you're as dumb as an ettin, a bunch of attackers doing the same things repeatedly is going to force them to learn to protect themselves.
So, if the same character is tracked doing the same thing over and over, have the game adapt to those tactics. If it's the 10th time in an hour that someone has gone into an area alone to beat on the poor melee creatures, have those creature use some of their loot that hasn't been taken yet to hire a mesmer mercenary. And have it spam energy drains and enchantment strippers mercilessly while the poor beasts pummel the botter to death. A live player will beat a retreat. A bot player will bite the big one.
And the more often that account is the target of the 'mesmer mercenary', the faster and more frequent it will show up - dumb ettins will pay for results. Bot-man dies over and over. No more easy botting.