Prevent Multiple Registrations from same IP Address?

[Thunderbird8]

According to vB's help in the admin section, that setting makes no checks for IP.

Normally, vBulletin will stop users signing up for multiple names by checking for a cookie on the user's machine. If one exists, then the user may not sign up for additional names. Note: This does not stop users from logging out and then registering new accounts.

It certainly doesn't do IP checks on registration in vB3 (haven't put vB4 in a live environment yet).

There might be an addon that will do this, however.