Why don't you make it so the thread can only be bumped to the top once per day max? That seems like fair compromise and would certainly defeat the purpose of the paid bumping whilst still allowing active threads to be bumped.
Then you can attack a competitor's topic by bumping it in the middle of the night, etc.
Maybe make it so only the OP can do it then so he could choose the time it is bumped. At least they have to log in and post once a day for it to be effective (or use a bump button if that's ever implemented (which would be a good feature for the Marketplace as well and save people and staff having to remove the bumps which is always very tedious)). I think this would be a minor issue though as the middle of the night is going to be different all over the world. I think making it so only the op can bump the thread once a day would actually make the system work because they can't pass it off to someone else and they have to be active if they want the thread active.