Most shotguns come equipped with a "bead sight," a small metal ball attached to the gun barrel with a threaded post to assist in aiming. A bead sight resembles the more standard "post" front sights found on pistols and rifles. You line up the two notches of the rear sight on the shotgun with the bead to get an accurate aiming picture. Removing these bead sights is fairly simple.
Unload the shotgun completely and set the safety to "on." This should be the first step in any operation dealing with firearms.
Plug in your hair drier or heat gun and use it to heat up the bead sight. You want to break up any adhesive or "lock-tite" that may have been used in installing the bead sight.
Grip the bead firmly with your vice-grip pliers and turn counter-clockwise. Most bead sights are threaded screw-in sights and will come out easily after a few quick turns. If the bead doesn't seem to be a threaded screw-in bead, use your vice-grips to pry it loose by wiggling it back and forth until it pops out. A flathead screwdriver may also help you.
If you are not comfortable using vice-grips and a screwdriver when working on a shotgun, you can purchase professional "bead sight installer/removal" tools.