Why not just attach a name, a reason, and a reasonable limit (a couple a day sounds fair). If they are a ***** it will be obvious. If they have posted, they will be negged, if not, they will be banned by the mods.
/edit
Which I guess is to say, why not get rid of bumping altogether ? A one line post to reposition a good thread is not such a problem. If there is no good reason, they will be negged.
