You just make it honorary for a non-citizen. Like Angelina Jolie DCMG or Bill Gates KBE. They can't call themselves Dame or Sir, though.

In any event, the bigger problem with Prince is he's dead. Most honours - with the exception of gallantry ones like the George Cross - can't be awarded posthumously.

They also can't be revoked posthumously, being deemed to have died with the holder, which is why Jimmy Saville's knighthood couldn't be rescinded.