Force-categorize posts by user roles

I’d like to be able to force posts made by certain user roles (let’s say Contributors) to go into a specific category. I’d also like to take this out of their control, so that when Contributors create a post, they either see this category checked and grayed out, or they don’t see the category listed at all and the entire process goes on behind the scenes.

I’m trying to do this because I’m trying to make a separate blog for this particular user role and I figured this solution would be best, short of doing a multisite installation, which I think is overkill. This way, I can have a separate page that is just a category query to return all and only posts made by Contributors.

Thanks for your help!