Spectral Pitch Display is actually hindering you, not helping you. It's not showing you the low-level sounds that surround the spectral parts it is displaying, so even if you could manipulate anything from this view, you would be being mislead. So it is a spectral frequency display, but one that's partially crippled.
The specific answer to your question is to switch to the Spectral Frequency view, and make the selection there using the Marquee tool. Then hit the Delete key, and that part of the spectrum will be deleted. But then you will almost certainly find that your phantom voice is still there, because voices don't simply occupy one small bit of spectrum - all you've done is removed the low frequencies, probably from both voices.
In fact removing a single voice in these circumstances is pretty difficult, even using software designed to do this a little better, like iZotope's RX. RX is better because it lets you make harmonic selections based on the fundamental you select, but even that doesn't help you separate two voices mixed together in a satisfactory manner either.
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