Is it permissible to prevent the Imam of the mosque from leading prayers after discovering that he has committed the act of the people of Lot, and is it obligatory to inform other worshippers who are unaware of his situation so that they do not pray behind him, especially after it is known that he has not repented and continues his actions and seduces young men?

You did well in preventing the mentioned person from leading the prayer (Imamah); perhaps that will deter him from committing evil. It is not obligatory to inform your friends about him, but he should be advised and called to repentance, and threatened with informing the authorities if he persists in his straying. At the same time, call upon him to abandon the Imamah entirely, as he is not fit for it. If his return to evil is proven with clear evidence, refer his case to the Sharia judge to administer the deserved punishment.