prevent inactive users from downloading

[smoovie]

I would love to see a hack that would prevent inactive users from being able to download content added to threads on my board. I know alot of my members register/will register just to download stuff and not post a single thank you. After the set time they will go inactive and just leech. Forcing them to contribute something would be spectacular. I hope someone else will have use for this and be encouraged to code it.