Blake's Adventure is a fun adventure game for one or two players. There are four worlds and sixteen different levels.
I'm sure there must be a more elegant solution, but off the top of my head I would: maintain the "Solid" behaviour for the boulder but add a condition where if "is overlapping" with Blake then "set Platform vector Y to -300" (or the number you think is appropriate). That should fix it.
Or you could just make boulders disintegrate once they touch the ground.