Or just simply make a subscription fee for all the users or just for participation in spesific forum sections like the bounty and signature once. So in order to participate in a bounty or signature campaign the user have to pay some fee to the forum and those money can be ivested in moderators. The high rank members can have lower fees, higher for newbies. It's just an idea. The forum is a grate place for me and I am willing to pay what it takes to make it even better.
If you believe a fee would discourage spammers you're wrong. It would only impact the economy as paid posting would become a little less profitable. Instead of 0.1BTC per month a spammer would be getting that - the fee, e.g. 0.08BTC. Still worth it! It would also discourage new users because not everyone is able to pay a fee from the start, while the long time spammers (who just have to be rich by now) would keep doing it anyway.