Jalil,
As Shaitaan pointed out pensions cannot be stripped willy nilly.
I think this has to pass through Parliament.
One can diminish or even do away altogether with pensions and facilities for future former Presidents by an Act of Parliament.
However, it is not possible for any Bill to have retroactive effect on Jagdeo, Hinds, and Ramotar. They're pretty solid for all time to come.
The only theoretical way to strip Jagdeo, Hinds, and/or Ramotar is by amendment to the Constitution in the hardest way possible. Such a bill cannot even be passed unanimously by Parliament alone but also requires a successful vote at referendum because it alters a fundamental right. So for all intents and purposes, there is no practical legal avenue to undo what's been done by any future Government of Guyana.
How about if they get convicted for misappropriation of funds and thrown in jail...
I would love to give you even the hint of a satisfactory answer my friend but alas the Constitution and two Acts are clear. There is no such possibility.
Even if that were possible, there is still the issue of the President's lifetime legal immunity for any and all acts done during his tenure as President. The courts are even barred from even entertaining such a case.