What is the ruling on the imam reciting the post-prayer supplications aloud, which disturbs latecomers, especially after he has been reminded that the glorifications are to be recited individually and silently?

What the Imam and his followers are doing is contrary to the Sunnah (definition). You must continue advising him by explaining the sayings of the scholars until he stops. If he does not stop, then report him to the authorities if he can be deterred, and by doing so, you will be absolved of your responsibility.