It really isn't easy to assign a 'point of no return' when dealing with something so complex. Any number of things could have happened that could shift the course of history. But what the hey. I'll try a few.
Carthage was screwed when the Numidians joined up with the Romans before Zama and most of the elite Carthaginian cavalry was kept out of the battle.
Epeirus went to crap when Pyrrhus died.
The Third Mithridatic War was the beginning of the end for Pontus.