How do you discipline an eight (8) year old boy?

My son tends to abuse the goodness and/or the privileges that are given to him by my in laws (who lives a few blocks away from us). He misbehaves and purposely not follow us when they're around. It's tough when my husband and i have to restrain ourselves from embaring him and hurting our son's grandparents if we reprimand him on the spot. But then, i on the other hand would rather take the risk of hurting my grandparents than hurting my son's growth. HELP! i need your opinion on this : )