The forum has a built-in capcha and I made some changes in the register page to avoid bots. But unfortunately there was at least 2 spammers every week. This is not much, but this is annoying. Anyway the users never saw anything, because I must approve the first post of a new user. I spent time to delete the first post of these spammers.
I think the spammers are humans. Now I do not receive more spam