Well the easiest way is to see if you can download the instrumental. Check on limewire or soulseek.
If that fails you can try any program with a fairly complicated EQ adjustment (I recommend SoundForge) and you can try and find the frequencies that the voice is at, and cut it out. But you will still get some vestiges of the vocal track because vocals cover multiple frequencies at different levels.
Lastly you could email the artist (depending how big they are, the smaller the better) and ask for the instrumental track for personal use.