There are three passwords associated with the Slingbox. The Slingbox has an admin and user password. Change those. The last password is your Sling Account, you'll probably have to change that too.
Guest: Gives access for viewing.
Admin: Gives access for viewing and configuration
SlingAccounts: Gives access for viewing, configuration, and auto-locating the Slingbox.
Make sure you change all three passwords.
Had/have similer problem and I did not even lose my phone. I gave up for now and unplugged my box. Just hope the person that was logging in will be gone when i need it again. Sad thing is Im an engineer for Slingbox's parent company.
My original thread..