As Mac said, make sure the diagonal cutters are rated for hardened (often referred to as Music Wire, or Spring Steel) wire, otherwise it won't cut it and will damage the cutters.
Another way to go about it is to use a Dremel ® tool and a cut off wheel, leaves a nice square end to the cut spring if you can hang on to the spring securely while cutting. The diagonals tend to leave a knife edge on the spring that requires dressing with a grinder, stone or Dremel® wheel.
No matter how you go about it, be sure to wear eye protection!
If you use the Diagonal cutters, I always get set to cut the spring, then drop a shop cloth over my hands before cutting.
That keeps flying pieces of spring from going everywhere, like into your face.