Eliminate lead vocals on studio recordings?

If you reverse the phase of a track and play it along with the normal track, that will effectively cancel the entire track, not just the vocals.
From the SHS 6 Help files:
 
Inverting the Phase of a Track

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A waveform's exact opposite is called an inversion. It is a shift of 180 degrees. A waveform and its inversion cancel each other out completely, so it is usually not desirable to have two track recordings of the same source if one is phase inverted. It can lead to reduced volume, lowered or distorted response in certain frequencies, or even silence in the case of two tracks which are exactly identical (i.e. cloned tracks).
Occasionally, for example when recording a source using two microphones, one of the microphones may be recording an inversion of the other, the resulting tracks may, to some degree, be cancelling each other out. SONAR allows you to invert the phase of a track to match another.
To Invert the Phase of a Track Open the Track view or Console view. In the track you want to invert the phase, click the phase inversion button .