I do exactly the same as you and DS still hits me. Mostly in frustration but sometimes just because he can. I try to turn it into "high fives" or I pin his arms down and blow on his tummy, which diverts him, when it's just highspirited uncontrolled limbs. When he hits me then he is on the floor, told not to hit, and ignored.

The one I hate is when I'm pretending to nap (or asleep) and DS is in bed with me - he crawls over and lifts up my eyelids! At least you can ignore the hitting a bit.

He gets the "naughty chair" at Nursery and is made to apologise to the child he hit, which works better.