tell her to knock it off
ask your parents about it
avoid the slap
wear a scary mask all the time
tape a fake hand to your face for free high-fives
carry a spray bottle and spray her before she tries to slap you
say "HA! I blocked your hand with my face!" extremely loudly at her every time she does it
crawl everywhere across the ground for one day
get a really annoying app that plays a fart sound every time she slaps you
get a really annoying app that plays a crowd laugh track every time she slaps you
fall over on the floor and pretend to have died when she slaps you
fill your mouth with water and the next time she slaps you you'll spit it out all over her
or do all the above and see how far you get through the list before she stops