rsbob 

How about stuffing some batteries down the seat tube and adding some simple wiring thus electrifying the bike so that anyone who touches gets a very unpleasant jolt - not enough to kill but to prevent them from touching it again. It could be controlled remotely or maybe with a switch hidden under the seat. No need to deface the bike unless they return with thick rubber gloves.

Or there is the highly controversial bike mine: https://bikerumor.com/bike-mine-thef...e-off-thieves/