Is it possible to poke your eye out by rubbing it excessively?

No. Nothing's impossible, but the eye is very well attached in the orbit, by both the optic nerve, 6 extraocular muscles and multiple soft tissue attachments. It take significant trauma to forcefully remove an eye. If by "poking your eye out" you mean just penetrating it or rupturing it, this is more plausible, but would still require significant force.
NO. Chronic and vigorous rubbing of the eye may lead to conjunctival irritation or keratoconous but it isn't possible to poke the eye out. Now, some psychologically disturbed patients may express self-mutilation but this is extreme. Rubbing of the eyes is sometimes seen in small children who cannot see well. Best to get an exam to determine the underlying cause.