Hi Otrrob50,
First of all have a look at our high pass filter. This would be the simplest method. You can access it via Effects -> Cleanup -> High-pass Filter. In the dialog box which appears you select the frequency below which you would like to attenuate.
If you need more advanced options then take a look at our equalizer via Levels -> Equalize. Here you can select between Visual/Graphic/Parametric equalizers which should give you very precise control over how the frequencies are attenuated.
Regards,
Ben