To remove a client from your list of coached athletes, click on the client's name in the People Library to generate the pop-up box of client details, and then click on "User Settings".  Within the User Settings window, click on the Account tab, then click the Remove Client link next to the "Client type".

Once you remove a client from your account, the client will still be able to log into their account and all their training history will stay with that account, it will just not be attached to the Professional Edition account anymore.