Prohibition (Must Not / Should Not)

Writing a rule, a warning notice, or a legal provision.

～てはならない vs ～てはいけない: Use てはならない. It is the formal, written prohibition and carries the weight of a rule that stands independently of who is speaking. てはいけない is the everyday one, used to children and in day-to-day instructions, and looks slight on a sign.

Saying something is wrong to do, without claiming it is forbidden.

～べきではない vs ～てはならない: Use べきではない. It is a judgement about what is proper, so it fits opinion and argument. てはならない states a prohibition that someone has the authority to impose, which is a claim you often cannot actually make.