If there were a spell that you could cast on a wizard to turn them into a squib, they'd cast it on Hermione. So I'm glad there isn't, even though that would certainly be convenient.

Let me think about this. You could definitely use a rune on Finnigan that would give him insomnia. But, eventually he'd go to Madam P and she'd probably give him a potion at least for a few nights. For that matter, if he scribed a rune for peaceful sleep on his own bed it would probably counteract whatever you'd done, so... hm.