Will a magnet loose too much strength it cut with a hacksaw? Can a magnet be cut with a hacksaw, by hand, without losing its essential characteristic of being magnetic?  I know i would have to be very careful with heat but I don't know how careful
 A: This would likely mess up the polarity of the magnet near the area where you are cutting. It would depend on several factors, namely how big the magnet is compared to the hacksaw blade (the bigger the magnet the more likely you are to preserve it) and what the magnet is made out of, for instance if the magnet is steel then it would definitely deteriorate the field significantly, but if it were iron and it would not be as degrading because the atoms hold their polarization better. It would have no effect on an electro-magnet. 
A: "It depends"
It depends on


*

*what is too much

*what kind of magnet

*when is the strengt measured

*where in the magnet

*which kind of saw blade

*whether the surfaces are sealed


I assume "too much" means "obvious by manual experiment". (That implies you can get two magnets that are seriously dangerous for hands, if big enough)
I assume that it is about strong neodymium magnets, because these are used often, and are the most difficult case. I assume NdFeB (Neodymium Iron Boron) magnets rated N52  plated with three layers, nickel copper nickel. These are the "normal" neodymium magnets.
First, it will not use magnetism while cutting, but it will use some because of corrosion later if not sealed. That may be not "too much".
So yes, in principle, but there are practical problems:
NdFeB is a ceramic, so a saw blade for steel is not useful. But a diamond cutting wire could be used, that is a steel wire with small industrial diamonds on it. (It is not as expensive as it may sound, roughly 1 USD per meter).
If the wire is ferromagnetic, it may be annoying to position the initial cut depending on where to cut.
The new surfaces are not sealed by the nickel copper nickel plating, and the unprotected surface is susceptible to corrosion, as it contains iron, the "Fe" in NdFeB. Coating with epoxy or so helps.
The maximal temperature the material can handle is 80 degree C. Ceramics are not very good heat conductors. Very slow sawing should work, but normal tool cooling like with a drill in steel, or just water cooling, is enough if careful. But if water boils, it's already too hot. The handling of saw dust is certainly a little annoying.  
But fundamentally, apart from loosing the material of the cut, there is no reason the material should loose any strengt.
