What is the ruling on an agent who does not give his principals their rights, claiming that a certain group within the family is not entitled to inheritance, contrary to Sharia and law? And is there a reported supplication for this situation?

The agent has no right to prevent his principal from accessing his right. If he does, the principal can take him to court or dismiss him and appoint another. Similarly, the guardian (wasi) is not permitted to dispose of the wards' property except in ways that serve their best interest, nor can he prevent a rightful owner from accessing a right firmly established by Shariah. If he violates the terms of the guardianship, he must be dismissed, and his matter referred to the ruler (sultan) to handle it personally or appoint someone else, for the guardian's management of the minors' property is based on their best interest.