I completely agree with Debi, in your reaction to it, if she starts to do it again. Do not ignore it, show her that it is not acceptable.
My son (4) saw another boy at school do this so he came home and tried it out. I immediately sat him down and did just what Debi suggested to you (heck Debi an I might be the same person HA) This is not acceptable and what's going on, lets talk. Also I reassured him that I love him and if he's hurt, that makes me sad, so I never want to see him hurt himself because that makes me sad. ( I think that got his attention more than anything, but every kid is different, find the angle that works for you) But do not allow it, stop it quickly and it will quickly go away. My son did it one more time and it was over. (You know those little ones always have to test the waters ONE more time to see if Mom is consistant)