On Friends who played the girl keeps hitting Joey?

You're likely thinking of the episode "The One Where Phoebe Hates PBS", where Joey, on a date with Kathy, gets hit repeatedly by her when he says something she finds distasteful. Kathy's actress was Paget Brewster.

However, Joey was also hit by Marisa Tomei in the episode "The One with the Cop". This time, the slap was due to Joey's role as an actor in a play where he was to smack his character's girlfriend.

So, depending on which incident you were thinking of, the answer is either Paget Brewster or Marisa Tomei.