As an extension of the 助動詞（じょどうし） (auxiliary verb)
べき, that is partnered with verbs to convey ‘ought to do (A)’,
べきではない (or べきじゃない) highlights the opposite, and means ‘ought not to (A)’, ‘should not (A)’, or ‘must not (A)’. Just like
べき, it is quite strong, and implies that doing (A) would be considered improper/incorrect.
べきではない (or べきじゃない) will be paired with the plain (non-past) form of a verb.
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濡（ぬ）れている手（て）でコンセントに触（ふ）れるべきではない。
You must not touch the outlet with wet hands.
どんなにいう事（こと）を聞（き）かなくても、子供（こども）に手（て）をあげるべきじゃない。
No matter how much a child doesn't listen to what you say, you ought not to raise your hand at them.
As with all other structures that use では, は may be omitted. However, as the presence of は strengthens the ない, the overall phrase will sound a little bit weaker without it.
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一人（ひとり）であの山（やま）に行（い）くべきではない。
You must not go to that mountain alone.
人（ひと）を傷（きず）つけるような事（こと）を言（い）うべきでない。
You ought not to say anything that could hurt a person's feelings.
The combination of
する and
べき is a rare grammatical exception in Japanese, in which two forms are both considered equally correct. Due to this, either するべき, or すべき may be used.
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テストの前（まえ）に徹夜（てつや）をするべきではない。
You ought not to stay up all night before a test.
くだらないことで警察（けいさつ）に電話（でんわ）すべきではない。
You must not call the cops for something trivial.