A small padlock through the chainwheel around the chain, perhaps?
(I'd probably choose a different lock; this is just one I had on my workbench. Hmm. And perhaps orient the lock inside rather than outside. Hmm.)
Note: the rear wheel can still freewheel and the bike can still roll forwards.
That's a pretty good lock, because of the curve, it'd be really hard to cut off. Great deterrent to the random thief.
But if it's a bike that's wanted, it'll still get taken. I always think of my mate's motorbike, alarm screaming as it was going away down the road in the back of a van.