Idea is really good one.
However, we can't stop spammers in this way. Lots of people are selling merit. Those who are here for signature bounty will manage to get required merits. It will increase merit abusing too.
Also, spammers with higher rank own a lot of alt accounts. So, it will be easy for them while people who don't have alt, will suffer most.
Finally, we are giving priority to the spammers again. Only they will be able to earn huge.