The GM hands may be tied if you are unwilling to follow the advice given first (lock the FC house so that non-FC members can't get in, put the offending person on personal blacklist).

Hopefully you've provided them with the name, location and times of offenses. 'Predatory behavior' is a useless quantifier if there aren't solid reasons (chat records, mostly) to back them up. Complaints from multiple sources (including name, locations, times and offensive behavior) are more compelling, so if you have witnesses, have them put in similar reports.

Be cautious, because multiple people reporting a player can be interpreted as harassment if the reports don't pan out.

Sorry to hear of your problems. Hope it can be resolved.