They can't do a thing. I can go to the webmail server, detatch my personal device from exchange and delete the account on my phone, go on the exchange server and block my account from even being able to use it on a mobile device..as far as they'd know it's just another personal phone unrelated to anything company wise. Even if they did figure it out..I'd do a very very hard factory reset
I can, however, remotely wipe a phone an account is attached to if there's a security risk.