It's a long shot here, but try blocking off the entire area under the
bed.  Maybe board up the gap from the frame to the floor.  Or use a
staple gun and staple chicken wire or hardware cloth to the outside of
the box spring so it hangs down to the ground.  I'm in no way speaking
from experience.  It's just a suggestion here that may or may not work.
Or hey, just use the space to store a lot of, "under the bed" storage
boxes!
 
I'm not sure if this will work for you.  If you use wire, poke the pointy
ends in, so you don't scratch yourself getting in and out of bed.  We did
staple chicken wire to the bottom of our couch and love seat to stop our
ferrets from getting inside, and that worked like a charm.  The look on
my husbands face when I first suggested it, (He knew nothing about
ferrets) was priceless!  He said, "We have to turn the couches over and
do what?!?  Why!?!  How come?  They did !?!  Are you sure?  How many
holes!!!"
 
Pam
[Posted in FML issue 4679]