Some cats seem to be able to get into just about anything! But I imagine it won’t continue to bother if it knows there’s no food in there. Although it might just be playing if it’s a youngster. With my boxes, which have internal passages, the cats needed to get at an angle to get in, so that with an obstacle to the side away from the internal passage (in my case an earth filled flower pot, so not easy to move), it made things difficult for them. But with something in front as well, that would help.
Is the rock having any effect?