All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the matter is as you mentioned that these stamps are with them, then they should give them back to their owner, as the Prophet said: "A person is obliged to return what he has taken to its owner." [At-Tirmithi] So, you may seek the help of the pious and righteous people who reconcile between people and whom you think could influence the one who took your stamps. If this is of no avail, then it is permissible for you to take the matter to any authority which you think can bring you back your right.
However, it is not permissible to describe the person mentioned by being a hypocrite, as it is dissoluteness to insult a Muslim, but you are allowed to describe him by being a liar and the like, if lying is indeed confirmed about him. But it is better for a Muslim not to insult or curse especially the elderly. As-San'aani said: 'The scholars differed in opinion about the permissibility of insulting a dissolute person with the sins he committed; most of them are of the view that it is permissible.'
Allaah Knows best.