some forums have a contact the Staff link in the footer so if you are not a member of the forum you can use that to report a member on the forum.
If you are a member i would just do as skouliki has said and just send a PM to the Admin / founder of the forum it then is down to the owner of that forum to deal with it as seen fit.
Sadly there is no other way to do this.
If this is now solved for you please press the mark solved button found at the top of this topic.