How do you know? Do they wear stripy jumpers and carry a bag marked swag? As I've said before, I'm not sure it's a good idea to spread these accusations on a public forum.
I've always used stripy as the spoken version- never striped- but I don't think I've ever had cause to write it down before, strangely enough. I knew after I'd written it you'd be all over it so I looked it up and the first few entries I looked at accepted both spellings, so I left it. I must admit, like Sakho, it doesn't really look right, but I might keep doing it just to be annoying.
Stripy Update I've just dug up an English dictionary published in 1959. In it, "stripy" is the only adjective listed. So, I win again.