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This is a little like putting a book out on a table with a big sign saying "Please take a look!" and then being horrified when someone picks it up and skips to the last page. It may not be what you'd expect people to do (and most probably won't), but you haven't put in place any measures to prevent it or (more importantly) asked people not to. If anybody is downloading your games through the right-click menu, they can't possibly know they're not "supposed" to do that. Some developers actually include Easter eggs that can only be found this way.

For what it's worth, I wouldn't download and examine one of your games now that you've said this here, but if it's important to you then I'd put a note in the description. Even then I think you'd be counting on people simply respecting your wishes rather than having any legal power to enforce them. After all, as William O'Connell says, everyone technically has to "download" the game in order to play it in the first place.