You mean where it asks for something like “(6+5)−4”? That does indeed not seem to work, because every time you enter the correct answer it says you need to enter the correct answer.
But I may have found a way to get through to them. Well, two actually. One is to disable Javascript in your web browser before opening their contact page — this will prevent the browser from running the scripts that check whether you’ve entered everything correctly, especially the captcha. (I don’t know what web browser you use, and so also not how to disable Javascript in it; do a web search for the browser name plus “disable Javascript” if you want to know.)
Anyway, I did just that and typed a message that their captcha is broken. On hitting the submit button, I got an somewhat confusing error message:
The Dutch text in all-capitals at the bottom says “YOUR MESSAGE WAS SENT!” which seems to contradict the English-language error message above it. But this isn’t important — what is, is that it includes the e-mail address the form actually gets sent to:
info@belkits.com
All you need to do is open your mailer and send a message to that address to get around their poorly-implemented (in more ways that one) captcha.