If you have access to the server-log, you may try to track down the spammer's IP and put him/her on the banlist. Hopefully he/she has a static IP - otherwise your odds are bad...
On the other hand if the the entries have the same origin - let's say throughout a couple of days - you may opt for rougher methods. You can deny access to the site based on the IP in your .htaccess-file. Just send the guy a HTTP-403 status code. Example (.htaccess):

# Block spammer-IPs
deny from xxx.xxx.xxx.xxx
ErrorDocument 403 /path/403_forbidden.html

If you want to block more than one IP, just add another line.

works only, if the spammer is using always the same IP, isn't it?

'my' spammer is apparently testing whole IP ranges for open web proxies.
If an entry to my forum is possible, a working open web proxy is found -
that seems the strategy.
I.e., I got every spam from a different IP, and even from completely different
IP ranges. Bad

If this does not help, most likely you would have to wait for the final 2 version of mlf. Though the CAPTCHA/IP bans/not accepted words are essentially the same - there's additional spam protection by Bad-Behavior and Akismet. Here we had some spam in the beginning, but right now we have about one entry/month with only the math-captcha active.

Sounds good. I activated also math captchas, so i will see...

regards,

N.