The energy required to break all the metallic bonds in a metal would completely disintegrate the metal.

The spin of individual electrons can be flipped. One way to do so is to shine circularly polarized light at the electron. Another way is to apply specific frequencies of magnetic fields to process the electron spin from one direction to another. People who do research in spintronics try to find ways to efficiently align the spin of all the electrons in a sample in the same direction, then use that spin to study interesting physics and possibly quantum computation (far in the future).

Here's more of the details, if you're interested... Electrons belong to a class of particles called fermions. One of the principles of fermions is that you cannot have two fermions in exactly the same condition at the same time. This is called Pauli Exclusion Principle. For example, all the electrons in an atom have to occupy different energy bands (the s, p, d, f states in chemistry), or have different angular momentum (whether total L or along one direction, Lz), or have different spin. We say that the electrons have to have different quantum numbers. In some materials, and in conventional ("BCS") semiconductors, each freely-moving electron is paired with another electron in an identical state except with opposite spin. The electrons aren't bound to any particular atom, but they do still have to obey the Pauli Exclusion Principle. So you wouldn't be able to flip the spin of one electron because there is already another electron occupying the same quantum numbers.

Metallic bonds are the electrostatic interactions between negatively-charged electrons in a metal and the positively-charged atomic nuclei. The only way to destroy them is to either make the electrons not semi-free, which would mean oxidizing the metal with a non-metal to create a salt, or heating up the metal to the point where atoms in the metal and electrons attached to them are no longer in contact with it, i.e. boiling it into a vapor. The latter is much more reversible, but much more difficult to do safely in a classroom (metal vapors are highly toxic, plus this is a *lot* of heat we're talking about here). One thing you might look up, though, is stick a penny into a candle flame, which will oxidize it making copper oxide salt (the dark green stuff on the surface of old pennies), and then connect the penny electrically to a metal with a higher electropositivity (like aluminum), then let the aluminum oxidize and reduce the copper. I'm not sure how much energy the reverse step would produce, though, or how long it would take, but you might look into it.

Electron spin is a quantum number. Electrons change their quantum numbers spontaneously unless they are locked into place in their orbitals in full electron shells or by covalent or ionic chemical bonds. If the electrons are free to move around in a solid (like a metal), then the solid will conduct electricity. If the electrons are stuck where they are, then the solid will not conduct.