I think it should be a problem with local boards individually. I haven't seen the same problem on my board, however, it's less active than others. Anyways, it's a pretty good idea. In my opinion admins should really think about it.
Spam happens to be on all boards of the forum,be it the normal board or the local boards..
And I am kinda lost with the OP's suggestions,are we to introduce a specific merit system that differs from the normal one on his local board??
That's asking theymos for too much and I doubt if it'll ever be implemented,you can help keep your board clean by reporting spam to the moderators on sight