There is a special tool for this job but to remove these bad boys I always
just pry on them with a tire iron until they pop loose (pry from above so they
don't bonk you in the head).  I have devised a tool using a steel plate with
two bolts and nuts on either end.  The plate goes inside the "C" of the clamp
and you loosen the nut on the bolts until you jack the clamp open.
I have also devised another tool to do this job which is more like a wedge
that you put in the clamp when it is expanded (you figure out how to open it-
I used a very small vise and put the ears of the clamp on the outside of the
vise and then opened it).  Becareful using this method as the clamps are
spring steel and when you tap the wedge out of the clamp it will go flying!
(kids don't try this at home!)
Have fun and be safe.
Ken Wilford
Van-Again
John 3:16