How to delete voices from .mp3 tracks (iow; how to create instrumental songs)

In Effects>Stereo Imagery, there's a Center Channel Extractor, and that's fundamentally the only realistic way of doing this. But there are several caveats.

Firstly, it only removes part of a stereo image, and if there's anything else in the same position in the stereo field, it will remove that too. Secondly, vocals tend to be dead centre in the mix, but often the reverb applied to them has a wider stereo image, so that even if you can get the voice out, you're still left with a ghostly version of it in the reverb, and there isn't a thing you can do about that. Thirdly, using this tool is something of a lottery; sometimes you can get a good result with it, and other times it simply doesn't cut the mustard at all. Invariably though, it works better with good clean signals - and you don't get those from a compressed MP3 file with critical band masking applied to it; it works much better on clean wav files.

Finally, I ought to mention that despite what you see on CSI, isolating one part of a mixed musical track really isn't something you can do; at best it's going to be a compromise, I'm afraid.