Our 2-year-old dog Murphy "bites" hands and feet, too (not super-hard in aggression, but just in play). He also bites shirt sleeves (while you're wearing the shirt). Yelling OUCH doesn't work too well, but getting up and/or walking away definitely sends the message. Sometimes we'll squirt him with a water-sprayer (for plants). This DEFINITELY sends the message, and it's much more humane than hitting.

good luck!