Thanks! Both accounts have been banned and their profile cleaned of spam.
Actually, I banned 40.000+ users based on profile information and visit history and post history. The forum creates a cache file for various stuff which includes the banned users, but this created such a large banlist that PHP runs out of memory. DOSsed by ban 
I now permanently deleted most of these accounts (the ones that have 0 posts) and hope that this solves the responsiveness.