Have you seen the new ad campaign for K&G Fashion stores? It literally cracks me up everytime I see it. I've searched everywhere online for a photo still of the final frame from that commercial, with no luck.
Please understand that I'm not condoning this kind of thing, nor am I suggesting that you DO this to a competitor's windows. You have to admit, though, that in the face of ripoff artists, copycats, and absurdly overpriced merchandise, it is an almost understandable approach: If your artwork or window displays or web site has ever been copied by a competitor, isn't this the kind of revenge you'd like to get?!
You could, of course, grafitti up your OWN store windows. If you sell fashion, maybe you could try a version of 'What Not to Wear' and show two mannequins: one dressed WRONG, then paint the 'big red circle with slash mark' over it. Have a second mannequin dressed in the latest style - and maybe that one has a big yellow smiley face painted over it. Just do the paint on the INSIDE of your windows, using a sponge-tip paint pen (the kind they make for school pep rallies and post-wedding car decorating).
Display ideas are everywhere, aren't they?! Or, is it just me?