I have vBulletin set up so that anyone can register, but that registered users have very few permissions. When someone posts something that is not spam, I promote them to a custom user group called "Trusted Users" that basically has all the permissions of the default "Registered Users" group.
I have noticed spammers (or spam-bots) inserting links everywhere possible in their user profile, and then submitting their user profile to the search engines. To prevent this, I turned off the ability for registered users to edit their profiles. In most cases this works, but I occasionally find that a registered user (i.e. untrusted) has managed to fill in their "Home Page" field.
Does anyone have a clue how they could be doing this? The "Home Page" field does not show up on the registration form...
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