1) Don't allow guestst to email users.
2) ACP--> Settings --> Options --> Site Name / URL / Contact Details, find the setting, Allow Unregistered Users to use 'Contact Us' ans set it to "No"
3) Your forum might have been compromised. Run the Suspect File Versions tool and look for anything suspicious, most notably, anything that says File does not contain expected contents. If there's anything that says File not recognized as part of vBulletin, that's normal, as it's from modifications you have. Just make sure all those modifications are modifications you installed yourself.
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