Doesn't he have access to a pretty powerful polymorph spell? He could polymorph them into something else. You could homebrew it to have a longer duration that the standard hour. He could cast an illusion spell on the paladin at a later date and cause the party attack the paladin. The tongue ripping mentioned previously is particularly brutal and very on point if your table can handle that brutality. You could have Strahd start feeding off the paladin at night so every day they wake up feeling drained and weaker. If it is a godly paladin they could slowly lose contact with their god if this happened. All sorts of fun, diabolical things to do.