Horsemom,
unfortunately Mimitabby is right, there are creeps everywhere.
Stalkers using public forums are not unheard of, and like any stalker they can be very dangerous until proven otherwise. The positive side is that there is professional help available for victims of stalkers (and for those who have reason to think they may become victims of a stalker), and support as well - so you are not alone.
My suggestion is to take stalking very seriously from the start (cyber-stalking included), and to begin familiarizing yourself with basic case management recommendations, and with resources and laws available in your State. I think the NCVC still keeps an updated archive of State laws, so you may want to take a look there, and maybe find out if any of the PD's in your area has a unit that specializes in stalking, or cyber crimes, or something similar.
If the subject keeps showing up on your forum and the inappropriate posts keep coming, I would even get a consultation about your case from a threat assessment firm. Most stalking cases escalate because they are not professionally managed; the victims underestimate the problem and try a 'do it yourself' approach without due knowledge. Of course they try to do their best to solve the problem - but if you are not a professional of threat assessment it is very easy to make things worse unintentionally.
Try to do some research on stalking management, get some good advice from the pro - and be careful.
A couple of last little words of advice:
1. identifiers on vehicles are often a no-no... even more so if you think you may have an unwanted pursuer - so I would remove your name from the bike, as well as things like personalized license plates, etc. It may be good advertisement - but it is never worth the risk.
2. it is nice of your forum's administrators to delete every inappropriate post (make the forum more enjoyable for the users), BUT remember that those inappropriate posts are EVIDENCE. So if you can, keep track of all of them before they are deleted. Most anti-stalking laws require proof of inappropriate communications, and in some States you need to prove that you had reason to worry about your safety (sometimes proving fear as a state of mind is enough, but not everywhere). In other places, proving that your pursuer had access to your whereabouts (for example he knows the trails where you ride and has been spotted there looking for you) is enough evidence for a restraining order or a warrant. It all depends on the case and on the evidence you have, so keep everything!
All communications have to be collected and stored and considered as pieces of evidence. We all hope that you will never need them, but it always better to collect evidence that you'll never need - than to need it and find out that you don't have it.
Take care and stay safe!