Jeremy Harris built an RFID system in the seat that does this. So if you're not on the bike it won't go.
You coudl make something to extend it's range.
Personally, if it's a thief, I'd have a second system that electrifies the handlebar grips with a conciousness-robbing shock, high enough voltage to even go thru thin gloves if possible, whenever your RFID is out of range. Just make sure you wear gloves thick enough to not be affected should something go wrong with the RFID.
Alternately, I found a doggy door by PetSafe that has an RFID "key" that opens when the key is within a preset range (it has a variable range setting, too, but it's not very far), and it can use up to four different keys, IIRC. I haven't yet converted it to the bike "ignition" RFID, but plan to when I have time. I got it really cheap on clearance, but you might find something similar on the web cheap, too.
It would mostly require adding a latching circuit that would convert it's momentary door-opening-motor-signal into a latch for when the RFID is near it, and unlatch when the RFID is pulled away from it. Runs off C-size batteries so you coudl power it off the bike's pack or it's own.
There are almost certainly aftermarket RFID systrems for cars, motorcycles and scooters, too, but I haven't looked for them.
Another deterrent system would be a remote controlled flame source, like a propane torch, that starts spurting fire straight up right in front of the seat.
If he's on fire he might not come after you.