I've just started developing my own usercontrols. First I tried editing the HelloMojoUserControl.ascx and was getting on well. However now updates don't seem to be taking effect.

I've only ever placed it on one page, but still cannot delete it even when it doesn't exist on the page. Here's the message I get..

This feature is being used in the content system. You must delete instances of this feature before you can delete the feature itself.

Thanks Joe, but I'm still missing something. 

If i follow your steps, it tells me there's no data, so it looks like it doesn't exist on any page. If I do a general search, I can see one instance of it on the first results page, when i click on publish/delete, it doesn't appear to be listed on any regions of any of the pages.