It would be against the principles of being a witness to exclude transactions from the blockchain. Going down that path would do a lot of harm to the network and token.
Other bot developers could hypothetically develop good bots to counter the bad bots though. There have already been a few cases of 'bot wars' that have played out on the blockchain.
That's all we need - a Bot War on Steemit! I only suggest that Witnesses could help to identify or "flag" the bad bots, not interfere with posting of content. Bots could be banned for innapropriate or bad behavior.