What not to do to get your site spammed?



If you don't want spam on your forum then you have to moderate your forum actively, you have to make quality posts regularly so that spammers wouldn't come to your forum, they would think that it is a quality forum and if I spam, I would be deleted.
 
Implement recaptcha is what alot of people do. Your site likely isn't worth the captcha breaking money.
 
On the signup form add some AJAX calls, javascript that needs to be processed for the signup to work. If that doesn't stop 99.99% of spam I will eat my hat
 
Require your users to have a minimum post count of 10 before they can post.