I am assuming you're looking to get the old Slingbox PRO-HD deleted from your account? If that is the case, you should simply log on to accounts.sling.com, go to "Slingbox Directory" on the left, and hit the Delete button next to the Slingbox you want to delete.
The above process works for older Slingbox models. If you are looking to delete the Slingbox 350 on your account, take a look at this article:
You'll find how to delete this Slingbox under the "What else does the Directory tell me?" heading.
Hope this helps!