How about we have Calvin pee on pictures of the people who make those stickers? 
I’ve seen a couple similar things with Simpson’s characters. But I thought Matt Groening has similar problems with people making stickers without his permission.
An actual question: If I am in possession of a product that’s “unauthorized” (say I am wearing a t-shirt with Mickey smoking some gangja), am I in violation of any law? I’m just wondering - always was curious about that.
BTW, Revtim:
You tell us, you’re the reverend. 
the ugliest one I’ve seen is fake Calvin peeing on the word “Japan,” which was written in rickshaw lettering.
I’m sick of them. The first one I saw wasn’t even worth a light “ha”. Now they’re tiresome and repetitive.
dantheman, it may be protected under parody laws, but IANAL. Maybe someone else can clear it up.
I don’t think anyone cares, but (again, my impression is that) Watterson would object to these violations not because people are making money off his product, but because other people are writing his character. Couldn’t they at least have used an actual image from the comic strip?
I think the earliest of these stickers (the Ford/Chevy ones were the first I noticed) did use an image straight from the strip.
I think it was a Sunday (color) strip that featured Calvin’s evil grin as he filled a water balloon from an outdoor tap. Calvin was hunched over the tap, with his hands in front of him. So adding a stream of urine did make it look like Calvin was relieving himself.
Later variations may have changed the image dramatically; I’ve seen some with Calvin’s pants pulled down, to make the point a little clearer, I guess.
Ooh! Ooh! I smell a Simpsons reference, but I can’t quite place it.
Well, I’ve always been tempted to make a Calvin Peeing on a cross. That said, I’ve dealt with people who get real close to violent over Darwin Fish, this would make them get their guns. My friend has also suggested Calvin peeing in Hobbes mouth.
When Homer’s brother was sitting around the campfire with the other bums (after Homer ruined him), one bum said something like “Yeah, I used to be rich. I owned Mickey Mouse Massage Parlors, until those Disney sleazeballs shut me down. I told 'em I’d change the logo, put Mickey’s pants back on! You just can’t reason with some folks.”