Still experimental, but worth a try is a filter by G3PPT.
Binster.zip (i quote) is
An audio filter program for a PC equipped with a soundcard and uses the FFT
(Fast FourierTransform) technique.
The incoming audio is sampled at 8000Hz and subjected to a 2048 sample FFT.
The unwanted frequency "bins" are set to zero and and inverse FFT carried
out to reconstitute the original audio minus the unwanted bits.
This has hitherto been difficult, probably due to the need for "windowing"
of the data before the FFT
to avoid spurious products with the result that one recreates "windowed"
The program is available at:
73 de Ko, NL9222