To proscribe something is to forbid or prohibit it, as a school principal might proscribe the use of cell phones in class.
Proscribe sounds similar to the word prescribe, but be careful: these words are essentially opposite in meaning. While proscribe means forbid, prescribe is used when a doctor recommends a medicine or remedy. Of course, if you want an excuse for not following your doctor’s orders, you could say you were confused about the meaning of these two words — but that would be lying, which is proscribed by most people’s value systems. And it would also be bad for your health.