The problem with what you have is the spammers are true humans not just a script.
So they pass any captcha or control method you may have implemented.

I have not tried it yet but one item you may try is the
stop forum spam mod at ubbdev.
https://www.ubbdev.com/forums/ubbthreads.php/ubb/showflat/Number/318736

I have used the service manually myself for years and it does a good job to track spammers.
I use it when I suspect a spammer registration. If they are in the stop forum spam database I ban then.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers