Maybe add a certain time window like 200 blacklists in 24 hours. I mean, if there are reports of a FC blacklisting another FC, there should be server logs of it and it can be used to punish anyone abusing the system. There will always be ways to manipulate the system, but a normal player playing the game and using features as intended should not affected by it. If you do something so lousy that 200 people come together and want to blacklist you - then you fudged up bad and need to rethink your play style.