A provider can always request it because she is unhappy with some reviews.  In that case though, the good have to come down with the bad unless she can convincingly prove to the management that the review was obviously bogus.

Also, sometimes providers retire, or have to lay low because of a stalker or some other dire situation.

Then there are providers who commit a cardinal offense and get banned, sometimes for a period of time and sometimes lifetime.  
This would be for things like posting a phony review of themselves or another provider.  Being too unfriendly on the boards or making vicious attacks on people is also cause to be sent to the corner.

For minor transgressions (like posting ads on the local boards more than once a week, or having a shill do so) will get you moderated where you have to wait up to a day to see your post set active.

I think that just about covers it all.