I don't think bans are given out for providing such services. Even if they could, your logic still wouldn't work here because they're huge account farmers and it really doesn't make a difference if one of their accounts get banned. Negative rating will stand as a warning for projects who hire these ANN bumpers that by using such services you're risking your project' future on bitcointalk.
The reason of my suggestion to give them a temp ban is this:
Sending unsolicited PMs, including but not limited to advertising and flood, is not allowed.
About hiring them: You think those who hire them really do care about their (ANN bumpers) reputation. They will hire the ANN bumpers just to bump their topics.
Since this service is aginst forum rules so it's not matter of just tag.
Not sure who gave you this idea,
1. No zero or low value, pointless or uninteresting posts or threads.
2. No off-topic posts.
3. No trolling.
4. No referral code (ref link) spam.
<and many more...>
Do we tag people for all above? Against the rule or discouraging to do something does not mean that you have to go and tag a user.