On my system, I unchecked the box that says to "remember my login and password" and my password was instantly removed from the sign in screen. The drop down box still retained all the logins I've used but the password was not automatically entered. You can give that a shot. If you want your login removed you'll need to click "forget me"
As for polygamy - I'm not seeing any options for that. When I try to login to a 2nd instance of WLM in Windows 7 it's not actually loading so I'd be forced to sign out of one and into the other account. Your's is letting you access more than one account?