Example: there are teenage gangs going around on high powered scooters in my city, carrying hammers and mini grinders. They pair up on a scooter, steal a bike and disappear.

I watched them. They don't want to hang around longer than necessary. They will only approach a bike rack that is clearly visible from the road. They will only steal a bike that has unobstructed access to the road (no tricky bollards or other bikes to get around). Even though they are full of bravado, and shout obscenities and threats at me when I tell them to fuck off, they still run away (even though the one approaching the bikes is carrying a weapon while his companion stays on the scooter ready to escape)

Anything that even mildly inconveniences these guys is enough to stop them attempting theft. The bikes they steal needs to be expensive, out in the open, with direct access to the road, and with a shitty lock. And believe it or not, those tumblers line up a lot.

Throwing a blanket over a bike is probably enough to stop them from even approaching it.