The following is taking from customer support (how about researching prevention methods guys?! It only took me a minute to find all of this...):
Most people are being hit with a lot of spam these days. Some things you can do to reduce this are:
1. Moderate new registrations:
Admin CP -> vBulletin Options -> User Registration Options -> Moderate New Members -> Yes
2. Activate email verification:
Admin CP -> vBulletin Options -> User Registration Options -> Verify Email address in Registration
3. Upgrade to 3.6.x if you haven't already since this includes an improved captcha for image verification. Then activate Image Verification if your server supports it:
First:
1. Ensure that you have uploaded the true type fronts from the vBulletin download package. They are found in the /images/regimage/fonts folder
2. Ensure that you have uploaded the background images from the vBulletin download package. They are found in the /images/regimage/backgrounds folder
Next:
Admin CP -> vBulletin Options -> Image Settings -> Image Verification Library
Set this option to GD (True Type Font). If the sample image shown below the option does not work, first ask your host if they can recompile PHP with Freetype 2 support. You want PHP to be able to handle True Type fonts properly and it will need this support to be able to do so. If your host refuses to do so or is unable to accomplish it, you will be forced to use the GD (Simple Font) option.
Your server may also support ImageMagick 6.0+. If you have confirmed this, then you may also choose the ImageMagick 6 option. It is as secure as the GD (True Type Font) option.
Next:
Admin CP -> vBulletin Options -> Image Settings -> Image Verification Options
Enable all options other than Random Shapes. If you are using the GD (Simple Font) option then you will need to enable ALL five options.
Next:
Admin CP -> vBulletin Options -> User Registration Options -> Image Verification
Enable this option.
You may also add your own fonts or background image to the system. Simply upload .ttf fonts to the font directory and .jpg or .gif images to the backgrounds directory (directories are detailed above). The backgrounds are used for all three image libraries and the fonts are randomly chosen for usaged by the GD (True Type Font) and ImageMagick 6 libraries.
4. Ban 'free' email domains many spammers use:
Admin CP -> vBulletin Options -> User Banning Options -> Banned Email Addresses
5. As a last resort, follow these instructions to
Add an extra question to the registration to prevent bot registrations. You should not need to do this unless you don't enable image verification or are forced to use the Simple Font option with image verification.
6. Some people have reported that spammers are using the 'Send Email to Friend' function to spam others from your forums. To reduce this make sure that your Unregistered, COPPA and User's Awaiting groups have the 'Can Use Email to Friend' option set to 'No'.
7. To help stop bots spamming through the Contact Us link, set this:
Admin CP -> vBulletin Options -> Site Name / URL / Contact Details -> Allow Unregistered Users to use 'Contact Us' -> Yes, but Verify Image