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What syntax are you using to censor the url?
Do you have protector installed? If so, which version? v3.16beta has some auto-registration-bot protection.
Also, do you have something like Captcha or Wishcraft's auto spambot protector installed?
Whilst we're on the subject; change the activation setting (Preferences>User Info Settings) to "Activation by Administrators" as a temporary measure.
In user settings, do you allow users to change their email address? If you do, try setting this to disallow it - you might be surprised at the results
Hope that at least some of this helps.