Just change the cookie name.
I'm talking on a user by user basis here (although the nuclear option of force logging everyone out is always good

)
I do think you can cancel all active sessions for a username by logging out, and logging in again. So the problem isn't as bad as one might think.
Huh, didn't know that :/
I still think this feature would be useful though for adformentioned reasons as well as manually logging in and out being a little clunky and non user friendly.
But regardless, I think what Aranator is saying is true. Maybe this is a symptom of a bigger problem.
If the cookie system ever does get reworked I personally think that adding in this kind of functionality would be a good idea.