Verify that the settings within your Control Panel are correct:
Membership Settings / Registration / "Require A Unique Email Per Account"UBB.threads
also checks for duplicate LOGIN NAME and duplicate DISPLAY NAME at registration, and in the PROFILE EDITOR for both the user side and in the control panel member editor.
IP Address blocking is not part of the USER BAN system. Though, it is part of the BAN LIST system. Meaning that when you ban a user, that user login name, user display name, user email address gets put in to a group with no access. The user can then go create another account with a new/different email address, and different names. The BAN LISTS are for widespread blocking of Email addresses and IP addresses in full, or with wildcards.
In Control Panel:
Membership Settings / Members / Ban Lists / "Banned IP Addresses" and "Banned Emails"
NOTE: The changes you make are "forward." Meaning that they begin at the time that you make the changes, and have no affect on past settings.