I think it should leave a ruined house in the spot giving negative prestige hehe. It can be built over with any other structure or deleted later if the square meets contiguity rules.

EDIT: Losing a food source causes a drop in population, houses to crumble and (with the prestige hit) a knock on effect of (depending on current prestige) losing even more population. It would then require building something else over it, pumping up prestige income via another structure or reclaiming the lost food source.

I'd be inclined to disallow removal of a ruin completely except via a new building. This would prevent the player's inevitable mechanic exploit of simply placing houses in non contiguous squares to begin with