Jonas Norberg has released Sniper, a plug-in designed to remove melodies from a mix, which it achieves by using a bunch of narrow notch-filters that are tuned to a note and its harmonics.

Sniper contains 64 narrow notch filters controlled by MIDI. By default no filters are active. When a note-on is received a group of notch filters corresponding to the harmonic content of the note is started. In special circumstances this plug allows you to remove or attenuate specific instruments from a mix while leaving the other virtually untouched.

It specifically solves the problem mentioned by Chris Huelsbeck in this thread here at KVR.

Sniper is available as a VST effect for Windows and is free.