The issue also is that many bounties are accepting newbies, even brand new users, in their campaigns.
That's why you need to introduce posting restrictions, or to ban bounties.
I think the only way to get rid of the massive spam is banning bounties. On the one hand we will lose attractiveness for those spamming bots but on the other hand a lot of good (in terms of helpfulness, posting quality, ...) posters will be lost, too.