The House of Windsor is screwed whenever the queen finally bites it; and I doubt parliament is going to be all that gung ho about finding replacement royals.
Only thing I can think of is to make it so the consequences of doing that are too dire for anyone to want to risk; because as it stands, there are none.
Then they'll just need to "prove" that he's guilty. Remember, we're talking about people who have no problem lying to the American people; even when it's blatantly obvious that they are. Because they always get away with it.