Thanks @wilfredn!

When I've looked at spam accounts before in analyses, the majority do tend to shut down once their potential return falls to zero. No doubt they start up again with a new account but that's a different problem to solve.

Another option (bouncing off the ideas in your comment) is to reduce the delegated SP or bandwidth as the debt builds up, so spammers spam themselves into submission.

To prevent free speech suppression it could be made possible for other users to remove a user's debt using upvotes. The amounts of potential debt would only be small, unless the user is producing vast numbers of flagged comments.