There are a couple ways you can delete the old Slingbox from your account.
1) You can use the directory at newwatch.slingbox.com to delete old Slingboxes. Here's an article that covers how (towards the bottom of the article):
2) For older model Slingboxes such as your old Slingbox SOLO, you can also login to accounts.sling.com, click on "Slingbox Directory" on the left, and delete it from there.
Hope this helps!