Use a pry bar, or a small crow bar to remove the nails. You can find these at The Home Depot, Lowes, or any decent hardware store. Just explain to a clerk what you need to do, and ask them to take you to the proper tool. Here's a link showing what some of them look like:

http://www1.dea…

When you have the tool in hand at the job site, gently insert it between the hanger and the wall as close to each nail as possible. Be careful doing this, so as not to damage the wall's surface. Place a small block of wood close to the bar to act as a pivot point and protect the wall from damage as you pry out the nails. As you can see from the link, some pry bars have a built in pivot point near their end, and may not require the use of a block, but it is still a good idea to use one because it distributes the force over a larger area, making the bar less likely to damage the wall. The block of wood will also make it easier to remove the nails, because it will act to raise the bar as it pivots over the top of it. After you have everything in place, slowly pry each nail in turn out of the wall. This will require you to also move the block from nail to nail as you pry. As each nail head rises from the wall, move the block closer each time you pry on it to gain the proper leverage to get the nail to move again. Above all, be patient. Getting too rowdy while removing nails from a wall can cause damage and entail even more work and time than the work and time spent removing them carefully. Good luck with your job.