vBulletin Manual - Human Verification Options

Use the vBulletin Options > vBulletin Options > Human Verification Options setting group to control where human verification is required on your forum in conjunction with the Human Verification configuration settings.

Verify at Registration
New users will be required to pass the Human Verification test during registration.
Verify Guest Posts
Guest posters will be required to pass the Human Verification test before their messages are posted.
Verify Guest Searches
Searches by guests will be required to pass the Human Verification test before searching is executed.
Verify Guest Contact Us
Guests will be required to pass the Human Verification test before leaving feedback. This only applies if sendmessage.php is being used as the Contact Us Link and Guests are allowed to use Contact Us in Site Name / URL / Contact Details

When you are done, click the [Save] button to apply the changes.