Good evening!
I don't have too much time recently. However, even after adding extra challenge to the new user registration process the bots kept registering - not only few fake users per day.
So I decided to immediately spent some time on the problem and - hopefully - found a good solution:
To register, you will have to negotiate a Captcha. After that, you will never again have to do that. However, this allows us also to enable anonymous posting in our forum again.
I really hope this will solve this problem for now and forever. However, we will disable anonymous posting as soon as our forum is misused again.
Also note, that currently comments need still to be reviewed by an admin if posted anonymously. However, if the captcha works well, we will have second thoughts about that, too ;)
Have a lot of fun!