How to configure Q&A CAPTCHA.
The Captcha which you are using to block spammers from registering is so old that all bots can easily solve it, and also humans who have no interest at all in your board, but for spamming.
Using a Question and Answer you can prevent automatic registration if your question requires some real solving.
NOT what is 2+3 because that is a) too simple, b) type it into google and the answer comes immediately and c) bots can perform simple algebra
Unfortunately I can't read your language but if you ask a question (in the Q&A) about your board and force some maniputation of the answer, then the bots won't be able to pass that test (for some time).
For example, you could have a logo with a couple of words in it and the question could be : what is the second word in the logo, spelled backwards with inverted capitalisation.
If you logo contains : My FaBuLouS Site, then the answer would be/ sUOmUbAf.
Granted, some people may have difficulty but then if they are really interested, will probably use the contact form. Bots won't pass that test easily and human spammers will in many cases think it's not worth the effort and move on.