They already kind of do that, mostly targeted at spam-following (following lots of people to get their attention for your spam). A large number of people blocki

They already kind of do that, mostly targeted at spam-following (following lots of people to get their attention for your spam). A large number of people blocking the user and/or having a high following/follower ratio (following many times as many people as follow you) get you looked at for spam. But for now, neither spam @replies (which happen occasionally too) nor spam #hashtags violate the TOS, I believe.