The rectangular stickers are perfect for this. You could also put them on shirts, I guess, and maybe certain posters, but I'm mostly thinking about the stickers. And the magnets, I guess....
The latest calling for warning labels on soda, along with taxes, because sodas are the 'cigarettes of obesity' reminded me that I suggested this a while ago.
I don't have any specific ideas right now, and I'm only assuming that the basic rectangular outline pattern isn't somehow in violation of some copyright...and I assume the word WARNING is still available for use....
I just think it's time to, y'know, start making more warning labels available. Because people need to be warned. About...stuff.