Software changes require admin or site admin privileges although you can grant a person certain access with some forum software. Prevention is mostly a admin duty unless its something like responding to an email to get registration/post approval. Ive seen very active forums use things including email and a certain post count in a "newbie" section before they can post anywhere else. If spammers are forced to post in a "newbie zone" first they are easy to spot.

No signature line until you reach 25 posts
No posting links, photos or videos until you reach 25 posts
No access to classifieds until you reach 25 posts
All of the above can be set to where a mod or more commonly a super mod must grant permission. The person just needs to PM or email them. It pretty easy to spot some guy that claims to be from NY but his IP comes back as China or India....This aint rocket science.

First 5 posts need to be in the newbie section before you can post anywhere else. (This can be setup through a software extension) Some forums call this things like "Introduce Yourself"