I have learnt two variants for prohibition: -ちゃだめ -ではいけません depending on the level of formality…
But does an intermediate form of -じゃいけない or -ではいけない
exist as well?
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Yes, all combinations are possible. Taking 見る as an example, you can say all of the following (roughly in the descending order of formality):
だめです may look like a weird combination of a colloquial word and the polite form, but it can happen when a teacher say "don't ..." to their students, for example.