Well, I mean having the steam overlay in software is kinda weird, but whatever. You can disable the overlay for a game in it's properties in your library.
Keep in mind I'm using a steam skin, so your library may look different.
Also keep in mind that the checkbox is there and checked by default on
any steam item. This does not necessarily mean the item supports the overlay. No one else has had the issue you're having, so it seems to be a very isolated incident. I'd be interested in seeing a screenshot of the overlay on the editor, personally, that sounds hilarious.