You can delete the Slingbox from the directory while on Watch on Slingbox.com. The option will appear in a drop down menu in the top left hand corner underneath the Slingbox logo.
Here is an article that shows how to do this:
Sling Media Moderation Team
Sorry to bring up an old thread, but I added a slingbox M1 hosted by someone else to my directory. I try to delete it using the method highlighted in the link above. It prompts me to confirm if I want to delete it, but them stays on the "Deleting Slingbox" screen forever. Is there another way of doing so?