What should I do about an Imam who frequently narrates strange and fabricated Hadiths in the Friday sermon and does not respond to advice?

Judging a hadith as authentic or weak is a matter of ijtihad (independent reasoning), and hadith scholars may differ on the authenticity of a hadith due to their differing views on the fulfillment of authenticity conditions, or on the necessity of some of them. If a hadith is declared authentic, it does not mean that it is definitively established. There should be no denunciation of someone who cites hadiths about whose authenticity or weakness there is disagreement. Assuming that the preacher cited very weak or fabricated hadiths, he should be advised gently and informed of the danger of attributing statements to the Messenger of Allah, peace and blessings be upon him, without knowledge.