Hello Dogger,
The best thing you can do is incorporate a CAPTCHA into your forum. To do this varies with the software you're using. Basically, it verifies if the person is a human or not entering the link. You can also BLOCK IPs if you want, but then ANYONE from that IP would not be able to get to your website. I'll post a link for both possibilities below:
What is a CAPTCHA?Block Country IP range
If you have any further questions, please contact technical support or leave a comment at the bottom of the page.
Regards,
Arnel C.