You could connect the metal frame of the sign to your doorbell, they’d get an electric jolt and might take the hint.
Or you could use high-strength fishing line to rig the sign to a paint sprayer, the next day tell the cops that your culprit is bright green (or whatever color strikes your fancy).
Seems I recall seeing ads for a motion-detecting lawn sprinkler thingy that squirted water from your hose at intruders (and rabbits, mainly rabbits).
How cruel do you feel like being? You could rig the sign to a gun loaded with blanks and scare the pants off him!