I'm assuming that you already have URL validation turned on, but you can't keep up with this? If you don't, then you might want to turn this all so that all links must be validated before they are published to the directory. However, this really won't help in reducing spam.
To do that, try turning on "CAPTCHA" so that anonymous users must enter a random code when submitting links. You'll find the option on the Wiki admin page and it is labeled "Anonymous editors must input anti-bot code:". Although this option is under Wiki admin it applies to many Tiki features.