Punkfolkrocker has it right. Order a "staple remover," a very small version of the basic pry-bar with two sharp prongs. It's strong and sharp enough to hammer in under the staple and pry it up with a minimum of damage. Artists who staple canvas to their stretchers use this tool all the time. You may even be able to pick one up at an art supply store rather than a regular hardware store.

If the staple breaks, which some do, I usually have to dig around it with a small chisel and extract it with a needle-nose vice-grips.

Sometimes I just take a grinder and grind the sucker flat.

Sometimes I just cuss at it.

Cheerily,
Charlie Ipcar