My suggestion would be to stop accepting everyone (and subsequently blacklisting the spammers), and switch to the system most Bitcoin paying campaigns use: only accept people after reviewing their post history. Or as I've suggested to Yobit before: if they really insist on automated enrollment: add at least an
earned Merit requirement. That would instantly disqualify most inactive and many spamming accounts.
Reset criteria to accept participants with their application is good, but there is still issue that how many earned merits should be used as additional requirements for participants as Whitelisting process?
In my opinion, requires 5 or 10 merits still does not enough to prevent spamming. Because if they exchanged merits months ago, they will easily to satisfy 5-10 earned merits criterion.
I suggest to require 20 earned merits at least with their applications on Yobit site. Anyone applies with less than 20 earned merits will be automatically rejected by Yobit system. After that, manager will manually check their posts and their post history hours later.