Put the ice box down near the wall (1 tile distance) and wait, the boulders como from back and they will stop, then you use the ice box as stopper to drag them into water (of coiurse you need to find the right place for the bridge).
Maybe you will understand it better if you take a look at this screen:
Actually, you use the ice block to stop the boulders - but then use the coins & empty tiles to line up the boulders to make the bridge. You need to time the last boulder just right with the ghosts' movement so you can run in & block them off!
Necessity is the mother of invention. I tried for a long time to block off the ghosts and never could. However, I found if you just run right across, the tile "breaks" creating water, which the ghosts cannot pass, so no need to use the block to control them.
You're right, Steve - I worded it wrong! That's what I meant by blocking them off - stepping on the water tile so they can't escape! Sorry for the confusion.