F7 is great for removing the UI for screenshots but the version number does remain.Try F7. I can't remember if that removes everything including that or if it only does everything except that.
I would assume that it is there so people know how dated an image is. Once 7DTD goes gold I'm sure it won't be there anymore.
I don't do screenshots except through my OS so I don't see the stamp you're talking about. But... if you turn off the interface is it still there on the screenshot?