We use pieces of wood cut the width of the open space between the
washer/dryer/refrigerator and the wall, high enough (so they can't jump
over) and attach it to the wall with metal angle brackets.  This blocks
their access from getting behind the appliances.  I know this is a better
idea for home owners than apartment dwellers, but it keeps the munchkins
out and safe.
 
Mary and the 11 Munchkins
[Posted in FML issue 2750]