If you are still having issues with your Sling Account, I recommend you to delete the temporary Internet files, history and cookies of the Internet browser that you are using (Firefox or Internet Explorer). Then, at the Slingbox location, run again the Setup at slingbox.com and confirm if you only have now the Slingbox that you own on the directory, if you have another one there, delete again. Finally, log out from the account and go back to the login page, there you should not see the other unit anymore. If the issue persists, I suggest that you reset the browser, you can more information here:
Regards and hope this helps.