If it's hardened (By now it probably has; if it hasn't then what kind of freaky foam are you using?), then why not just sand it down? Unless you didn't get a lot of foam on you, in which case it shouldn't be a big problem for you. If you don't want to sand it, then go run your hand on something coarse for a couple hours. If that doesn't work, then I'm out of solutions. Well, solutions that'll let you keep your whole hand.
I'll probably take a reputation hit for that joke. Ahh well...