Many cable companies cut and fuse cables in a single operation using electrical resistance heating.

The cable is clamped across a pair of copper connectors with a small gap. A current is passed through the cable heating it at the gap, and after a second or two it heats red hot and vaporizes leaving fuses ends. Campagnolo does it slightly differently, when the cable is melting the two electrodes are moved apart pulling the cut and fused ends into a point.

On my own bikes I use a small oxy-acetylene torch with a reducing flame which rapidly heats and flame cuts the wire, leaving perfectly fused ends.

I'm a fan of fused ends because it allows the cable to be pulled for service, and reused.

Others have reported good success with super glue or similar applied before cutting, but this only works with brand new wires that are oil free.