I only had that happen once when I was getting the kids up from their nap. The child was about 22 months and being in the room I caught him in the act and gave a very stern NO with stern facial expression. He didn't try it again.
I think she is too young for a mat IMO so you may have to invest in monitoring system to keep an eye on her. If you can catch her in the act and discipline at that moment, it should deter her (hopefully)