Can't, not allowed いけません/いけない

This is how to say something is forbidden to do. Perhaps easiest way to us this is to stick it after the て form of a verb and は (wa - topic particle).

知らない人と話してはいけません。
shiranai hito to hanashite wa ikemasen.
Don't speak to strangers.

Often in casual speech, the ては becomes ちゃ (or じゃ) as in:

その映画を見ちゃいけませんよ。
sono eiga o micha ikemasen yo.
You are not allowed to watch this movie - or You shouldn't watch this movie.

You can also use だめ for a similar effect: 私の本を読んじゃだめ。watashi no hon o yonja dame. You can't read my book!