Is it possible to de-auth a client, and then make sure they have to enter the passphrase if they want to connect again?
I just cant seem to find this option anywhere in my routers configuration page.
You can deauth it, kick it off, but you cannot force the users OS to forget the saved password (atleast not if you only have access to the router - if you have access to the PC, you can do whatever you want with it).
A (very bad) workaround would be to change the password temporarily, so windows thinks it's a wrong password when it reconnects, and then shows the password prompt, after which you change the password back. But that would also kick off all the other users on the AP.