べきではないShould not, Must not , Ought not to

As an extension of the 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.

Caution

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.

Fun Fact

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.