They will be forced to stop when it becomes more popular anyways, unless some sort of lottery systems gets introduced or fees get bumped up into a $1000 range. But even then I think most will pull out of it after a while, especially if it gets popular, hard to imagine it is an exciting activity to do hundreds of such video messages every day.
They could add a "lottery" element to it. A less evil implementation of that would be to ask the performer what size stack they want and just keep a list of the latest n requests (or requests up to m days old). Automate a reply "sorry you weren't able to get a message from $celeb, blah". You'd need heavy filtering to avoid abuse (automated re-requests).
