This reminds me of living in West Oakland in the 90's. I used to get my battery stolen for it's lead value on a regular basis. Finally started removing the battery every night, man did that get old! I'm surprised the crackheads of the day didn't figure out this little trick. But then that was the nineties, economic boom times for drug dealers compared to today's drug-war salvaged landscape and crack pipes were much cheaper and easier to come by. So really you can't blame them for getting creative
take them with you!
Not really a practical solution, how would your plugs (let alone your cylinder head) like daily removal and installation? Maybe try collecting dead spark plugs and leaving them propped next to the ones in your engine. Presumably they're just after the insulation and don't actually want to vandalize your bike. I know, I know, probably a 50/50 chance at best.
Good luck!