How can the National Diet of Japan be prevented from enacting laws that violate the Constitution?

Today, I studied the Constitution of Japan and noticed what seems to be a significant issue. While the Constitution grants the Supreme Court the authority to determine whether a law is unconstitutional, it does not empower the Supreme Court to directly nullify laws it deems unconstitutional.

Consider an extreme example: if the National Diet, under the control of the Liberal Democratic Party, were to enact a law prohibiting citizens from criticizing the party, such a law—being a clear violation of freedom of speech—would almost certainly be ruled unconstitutional by the Supreme Court. However, in such a scenario, how could this oppressive and unconstitutional law be repealed? What if the Liberal Democratic Party’s legislators refuse to overturn the unjust legislation they themselves enacted?