Robette: I don't think they can, after all you can still download games which you bought but which where subsequently removed from the store over copyright issues or the likes.
firstpastthepost: I'm sure they can be removed from our accounts. I'm positive of that actually. All it takes is a letter from a lawyer from the rights holder. GOG doesn't own the products it's selling, if they're told to purge the data they hold they would have to comply or go to court.
GOGs terms and conditions to us can't override the terms and conditions of the product they are selling, and those terms say we are purchasing a license to use the product, not ownership of it.
It is possible for them to do that, people make that request from time to time.
Legally though, as long as GOG had a license to sell a license of the game at the time that the sale was made, then there's nothing that the rightsholder can do to force the game's removal.
The only exceptions I've heard were when it turned out that all the rightsholders hadn't signed off on the plan because there was somebody that wasn't known to have rights related to the product.