I am trying to demagnetize this vise grips.
They became magnetized because of repeated times that small magnets on my work bench stuck to the vise grips.
I tried the method where you hit it with a hammer, but it does not work.
There are a few ways to destroy a magnet.
Beyond the great answer by Stan, there are actually a few tools that'll demagnetize things. Googling "demagnetizer" seemed to bring up a lot of good options, with a lot of places to get them, including the standard online, department, and home improvement stores.
You can also get an electromagnet to do the same. The below is a random example that happens to demagnetize watches, but there's others out there, including ones that do old-school CRT TV screens.
My father did it many years ago at home with old soldering gun that he always had at home. It was very easy:
How it works: the rear part of the soldering gun creates a magnetic field of periodically changing polarity (it's under alternate current). As you slowly move the item away, it's magnetic field will be constantly oscillating with lesser and lesser amplitude, so it will basically converge to zero :-)
PS: the image is just for illustration (not our original soldering gun :))