Depends. With some work, you can create a self-policing system. If you "fix" prefix-less threads, and post on those threads saying why you did it, you should be able to build up an expectation amongst posters that this is the way to do it. After a while, you'll get posters telling each other off for not using prefixes and your work load will drop off to a low level.
Agree with this. I believe most of our users will not use the prefixes if they are not required. Please give us the option to force users to select prefixes. I need this in my buying/selling forum. Waiting for this to be implemented in 1.1 beta 3.