In any event, I feel the pain. I've got a forum that targets hardcore salespeople and I've got a similiar rule. Except I use URL drops as carrots. I don't allow self promotion until after you're established.
You want a link drop? I want some quality posts over a period of time. Some of my best posters started out as drive by URL drops. They get a PM from me, they reform, and by the time they're able to link drop, they're getting business from other members of my site - because they've established themselves as regulars and experts.
Even then it's still tough to moderate and I have to make choices. Someone drops the minimum required posts, then the very next one is a mile high promotional ad - delete it? what if that same ad is then commented on positively by long time members of the forum? I know darn well that the poster just put in his time on the forum to post their ad and I'll never see them again - so I'd like to delete. But other members of the site like the post, so it stays.
danged if you do, danged if you don't!