If they crack one they can crack another too. Is it possible to take the submitter(after the submission) to another page "e.g. thankyou.tpl" [Thanks for submitting, if you want to submit again, browse the appropriate category and click the submit link] It will display the message for 3-4 seconds, then redirect to the homepage. In this way one can avoid repeated submissions.
I will stop accepting link exchanges all together soon. I only do them on two of my sites now and that will eventually just become none. I don't really think link exchanges are worth it anymore where I allow people to add any type of site to mine if they link back. I will keep the link exchange on my affiliate programs directory though, which means that if they add an affiliate program to my directory then they have to link back on a page that also links other affiliate programs directories or they must list me on their affiliate information page or even their homepage. I get some good quality link exchanges here, but then I also send alot of traffic their way, which is targetted.
Turn on captcha, and have a high minimum description text requirement like 200 characters. Spammers are not good at description writing. 99.99% of spammers fail this as it wastes a lot of their time on multiple submissions when they can't think of anything original to write. I even allow html deeplinking and they don't spam me.
It would be a good idea to release a variety of different types of captcha. If you release a trivia-based captcha that allows people to write their own question/answer combinations, that could be an effective approach. I know it's worked well for me. For smaller niche directories you don't need a captcha to be 100% uncrackable, it just needs to be unique. So that in the time it takes to break the captcha, you could have submitted a couple of sites already.
i had the same problem with a guy, and i emailed him, told him i am gonna block his ip address, and it stopped lol. but thats one reason, that now i am not going to make anymore free directories.. Just keeping the ones i have, and will turn them paid, once get a PR