If you're using the database driver for sessions, it's simple:
DB::table('sessions')->where('user_id', $user->id)->delete();
With the file driver it's harder. There's an Auth::logoutOtherDevices() method, but it doesn't work here because it requires the user's (unhashed) password. It works by rehashing their password, which makes other sessions fail the auth.session middleware check, effectively invalidating those sessions.
You might be tempted to add a boolean flag to the user model (e.g. require_reauth) which would be checked in a middleware. It would force a new login and get cleared after a successful login. But that won't work if the user is logged in on multiple devices.
You'd have to do a combination of those two methods: require a new login using the flag, then invalidate other sessions during the login with the Auth::logoutOtherDevices() method while you still have the user's password. Also you'd need to use the auth.session middleware in your routes.
I use DB sessions for reasons like this. If someone knows of a more convenient way with file sessions, please share.