Can Relic of Progenitus be used to prevent a card being fetched from the graveyard?
Planeswalker abilities are activated abilities; they go on the stack and can be responded to with instants, or other abilities which can be activated 'at instant speed', like either of Relic of Progenitus' ones. So that would work to stop them from using the fetch land twice.
Note that returning the land to the hand is part of the resolution of the effect, not the cost. This is different from the situation where they would activate the fetch land and sacrifice it as part of the cost; then, instant-speed land destruction would not work.