Welcome to WebmasterWorld Guest from 188.8.131.52
what software are you using now to run your directory?
Something I wrote myself. This is by far the best option, but if you can't manage this then I strongly suggest you find a script that not everybody uses, and one that you can customise as much as possible.
There are scripts out there that allow you to add custom word filters, and captchas. But if you use an unmodified form of, say, PhpLD, then you will get exactly the same spam as most other directories.