Well problem is most of the time they proxy through some unknowing unprotected users computer......so you can never find out who they really are...that's why you should always have a firewall to prevent the use of yours for bad things by others.
If they are stupid enough not to use a proxy, then you can trace them back, and report their details to their ISP, and depending on their ISP is either nothing will happen or their internet account will be restricted/closed.
Problem is they can cry wolf and claim they been a victim of proxying themselves though their own stupidness...and then the ISP is powerless to do anything.
And if your talking about ISP abroad, they won't want to know most of the time.
Key to stopping it is put checks in place first before someone can post on the forum..usually the effort of waiting to be vetted is enough to put people off of spamming your site and moving on to someone elses...if however they prove to be more persistant then you can ask them a question before joining like "tell me about your machine..."....most spammers wouldn't even have a clue about bikes so you can catch them out that way.
