No fun trying to stop bots. Maybe a filter could be set up for first time posters to filter out first time posts if there is a link in the first post, the post/account is rejected.
When registering first time on a forum, most forums send an email with a link to activate the account. In the email you are also advised that links, photos cannot be posted until at least 2 posts are made or you have introduced yourself.
Both car forums plus others that I have joined requested this. On one or two you could not post until you had posted introducing yourself. Never seen spam on either although it may happen.