In my experience, this solution is not so simple. To put it plainly, spammers have the advantage of automation and scale and easily wear out moderators.
A moderators job becomes picking out the one legitimate message out of hundreds and hubdreds of spam messages and approving it in a reasonable time scale. This is tiring and it's too easy to let someone's legitimate message go overlooked.
I, personally, don't have the free time to do this task well, but of there are willing volunteers for moderating, I'm open to trying it out.
--Tom