In general: don't use client variables. And if you do, absolutely DO NOT store them in the registry.
I'm pretty sure CF will sweep expired client vars if it actively notices they've expired, but if they get abandoned then it won't.
Anyway, I'd not worry about that whilst you have a server going crazy. Just delete them.
But also: switch client storage to use the DB, and then find out why yer creating client variables in the first place, and stop doing it.