What about a very simple solution such as not allowing signature space until an account is 1 year old. If you put a time limit on it rather than an activity limit, then there would be no incentive to spam (until the year is complete). If a year sounds too long, it could be 6 months. I think this could deter people that don't want to wait to get a signature.
They would just buy an account that is older than one year and spam on that.
Here's an idea that could work but probably won't happen because of the complexity:
In order to have a signature, an account must be, say, at least full member and the user must give 2
BTC to the forum. However, instead of that 2
BTC being a donation, it would rather be a security deposit. After the user posts at least 1000 posts over the course of two years, he can get 1.5
BTC back, the remaining 0.5
BTC becoming a donation. However, in order to get that 1.5
BTC back, the user must also not have more than 25 posts deleted over the two years, and no more than 10 instances where more than 20 posts are made in a twenty four hour period. He must also never receive a ban within those two years. Only when these requirements are met can the user receive the 1.5
BTC back.
I think this would work to reduce spam, increase the constructiveness of posts, and also maintain traffic for the forum. Users are incentivized to stick around for a few years, and post constructively in order to get their money back.