An electromagnet cannot usually stop a bullet as most bullets are made of non ferrous materials that the magnet including electromagnets don't attract.
If the bullet was made of ferrous materials or ferrous metals then the magnet could stop the bullet if it were powerful enough.
Magnetism is also canceled out by alternating current or any magnetic shielding material.
Increasing the temperature of the magnet or hitting the magnet also cancels out the magnetism.
Alternating current (AC) can be used to demagnetize a magnet and involves passing through AC through a coil placed near the magnet.
The AC current generates a magnetic field that cancels out the magnetic field of the magnet, which demagnetizes it.
Magnets are made of ferromagnetic metals like cobalt, nickel and iron or even alloys of these materials that are mixed with rare earth elements like samarium or neodymium.
The most common types of magnets are permanent magnets that are made from alloys like Neodymium Iron Boron (NdF-B), Aluminum Nickel Cobalt (Alnico), or ceramic "ferrite) materials.
What you should not do with magnets are drill into the magnets or cut the magnets and you cannot generate free energy, create perpetual motion, magnets don't stick to everything and magnets cannot maintain infinite strength or exist as a monopole.
If you cut a magnet in half, you will get 2 smaller magnets with both a north and south pole and not just one pole.
And while magnets can last a very long time, permanent magnets can also lose their magnetism over time as a result of improper storage, damage and eat.
And a system of magnets will also always come to rest as magnets cannot drive themselves continuously and magnets cannot produce any net energy output and the energy required to remove an object is also equal to the energy that is gained by it's attraction.
You cannot not drill a hole in a magnet as magnets are extremely brittle, hard and heat sensitive.
If you drill a hole through a magnet it may lose magnetism but mainly due to the heat generated by the drill bit when drilling the magnet.
And neodymium magnets also often have a surface hardness of RC46 which can ruin standard drill bits.
And the process of drilling a hole in a magnet can also cause the magnet to shatter, catch fire and lose it's magnetism.
Drilling a hole in a magnet also often results in chipping and cracking of the magnet.
The reason it's so hard to drill through a magnet is because of brittleness and fragility.
Magnets, especially the neodymium and ferrite magnets are brittle and can also break very easily and even shatter when you drill through them or saw them.
High heat during drilling of the magnet can also demagnetize or weaken the magnet.
So while you can drill a hole in a magnet or saw through a magnet it's not recommended.
A magnet's magnetic field can also be demagnetized by reversing its polarity, this can be done by placing the magnet in a magnetic field that is opposite to its own.
The opposite-field cancels out the magnetic field of the magnet, which reduces its strength.
There are a few ways to remove a magnetic field from a permanent magnet.
One of these methods requires increasing the temperature of the magnet.
Another way to make a magnet lose its magnetic field is by hitting it.