There's a really, really, REALLY easy solution - immediate suspension of any account that receives a set amount of blacklists. That suspension can be reviewed at leisure to ensure it is for a valid reason. If the blacklisting was malicious the offending users would get a warning/suspension/ban as appropriate. I believe that in 99.99% of cases the suspension would be valid & made permanent. I'm not talking suspension after 7 people of a Full Party blacklist, I'm talking about 50 users or so blacklisting someone in an hour or whatever.