When you were installing your toilet seat, you may have tightened the bolts with a wrench or a pair of pliers, and now — after several years — you should be able to remove it the same way. If only ... ! The humidity and lack of ventilation behind the toilet have probably corroded the bolts, and removal will likely be a chore involving spray lubricant, locking pliers, deep socket wrenches, hot putty knives or drills.
You Caught a Break
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In the fortunate event that you installed your toilet seat with plastic nuts and bolts, removal won't be much of a problem, but it still won't be as easy as it was when installing the seat. To avoid stripping the threads, spray the bolts with a soap solution before unscrewing the nuts.
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Some nuts have wings that you can grasp with your fingers; while turning these, stabilize the bolt from above with a screwdriver. If the nuts don't have wings, lock a pair of locking pliers around each one in turn. When you unscrew the screw, the pliers will hit against the toilet bowl and prevent the nut from turning, and it will loosen.
What a Day for Corrosion
It's no time for daydreaming when you're trying to get a stuck nut to spin on a corroded bolt, especially when you have to work in close quarters under a toilet. The locking pliers strategy usually works in this situation as well as it does when the bolts are plastic, but instead of using soapy water, Arco Plumbing suggests lubricating the bolts with spray lubricant.
Wait five minutes after spraying; then lock the pliers onto one of the bolts and find the longest screwdriver you can that fits the slots in the bolt head. You don't have to worry about holding the pliers, so put all your effort into turning the bolt counterclockwise with the screwdriver to loosen the nut.
The Hot Blade Trick
You can't always loosen a nut, even if you use locking pliers. Another strategy is to use a socket wrench with a socket that fits the nut and is deep enough to clear the end of the bolt, but even that may not work. If it doesn't, and the nut is nylon, you can melt it off with a hot putty knife.
Wearing protective gloves, heat the blade of a metal putty knife red hot with a propane torch and push it up into the nut, at its side nearest the bolt, from underneath. After it has melted its way through the nut on one side of the bolt, change the blade's orientation and make another groove on the other side of the nut, so that it can fall freely away. Repeat the operation on the second nylon nut and lift the seat off.
The Heavy Artillery
When it comes to removing toilet seats, the big gun is a drill with a 1/4-inch drill bit. Put on goggles to protect your eyes; then insert the bit into the bottom of a stuck metal nut that you can't turn at the point at which it meets the bolt. Drill a hole straight up through the inside edge of the nut.
Remove the bit and drill another hole 60 to 90 degrees away, then tap the section between the holes with a chisel, and the nut should separate. If not, drill more holes between the two you just drilled. After tapping off that section of the nut, you should be able to pry the nut off with a chisel or unscrew it with locking pliers.