Inspect the window frame and determine how the bars are installed. Most use one-way screws at least four inches long and ¼ inch in diameter. Most hardware stores sell a particular screw-removal tool designed specifically for burglar bars. Measure the screw heads that hold the bars in place before going to the hardware store, as the tools come in several sizes.
Make a rubbing with a piece of paper over the screw head to match up the screw size with the proper extraction tool. If a screw-extraction tool is unavailable, most burglar bars can still be removed using common household tools and, in some cases, an electric grinder.
Spray each screw liberally with a silicon-based lubricant to loosen the bolts and remove any accumulated rust. Allow the lubricant to work for several minutes.
Clamp down on each screw head with the locking pliers, then turn the pliers counter-clockwise like a wrench removing a bolt. The goal is to back out the screw completely. If the screw will only come out part way, or the screw head breaks off midway and the burglar bars will not slide over the bolt, use a Dremel tool to notch the bolts. The bolts can then be removed with a flat blade screwdriver.
Use a rotary angle grinder or hacksaw to cut off the screw heads so that the burglar bars can slide over the bolts and be removed from the window frame if a Dremel tool is unavailable or the space is too tight.
Cut away the screw heads and slide the burglar bars over the screw bolts to remove the bars. Any remaining bolts in the window frame can be extracted using additional lubricant and locking pliers, or the bolts can be ground down flush with the window frame.
Fill the screw holes with cement or wood putty, depending on the window frame materials. Smooth out the filling material with a putty knife and allow to dry.