Image BBCode Exploit of Sorts

[Andrew]

On a forum that I'm managing, I'm having issues with a user posting links to the member's areas of porn sites in the URL portion of the image BBCode. Basically, he loads about twenty into his posts, and then anytime anyone loads the post, they're prompted to enter a password to a porn site twenty times (You can see how this adds up quickly with multiple posts in a single thread.) IP banning him isn't working, because his ISP gives him a new IP whenever he restarts his router, and I can't disable images altogether for obvious reasons. Would there be an easy way to filter links like that from being inserted into the image BBCode ? For instance, he's not linking to an image, just the URL to the backdoor like http://members.whatever.com. I guess I'm really wondering if there's a way I could force the image BBCode URL's to have an image type file extension like .jpg, .gif, or .png ?