I too agree with the stand up and move away, maybe explain to them that it hurts your feelings.

I think this is really one situation where smacking them or hitting them back doesnt work because your saying dont hit its not nice and then you hit them back.
My DD sometimes hits me (not all the time, its an occasional thing) but i tell her straight away that you never ever hit anyone and when she hits me it hurts my feeling (sometimes with a fake cry) and i seem to get the sympathy and 'im sorry mummy'
As much as i just wanna smack her back sometimes so she knows how it feels its def in my opinion the wrong way to go.

Good luck to all you mums, i hope they stop hitting soon.