Plastic bolts too tight

I'm trying to replace two toilet seats installed by the previous homeowner. Both are held in place with plastic bolts/winged nut but I am unable to loose them even a little. With a flat-head screwdriver from up top and pliers pinched on the nut underneath and maximum elbow grease, I've only succeeded in starting to strip one of them. Even trying to move the nut has proved fruitless.

It feels like they are all deadbolted in. Can anyone offer some help, it would be greatly appreciated. This should be a five minute job, but it has me cursing up a storm!

Did the new seat come with new ones? If so, don't worry about it, either keep torqueing them until they break, or cut them off (need to be careful not to scratch the toilet, though). They could have used some thread locking compound on them, as those sometimes are hard to keep tight otherwise.

More often than not is is easier to hack saw the seat mounts horizontally just above the bowl, using care not to touch the porcelain.

Just drill down the center of the bolt with a 3/8" or slightly larger drill bit and the nut will fall off with the bottom of the bolt.