If you want to remove some peak artefacts from the signal, you can use the “Signal tools/Remove peaks...”  function. In its settings dialog, you can specify which peaks to remove, by defining their relative amplitude related to the standard deviation of the signal.

All values above this threshold will be removed by setting them to the mean value of the signal.

If you want this operation to be performed before each FFT calculation, you can set it in Spectral analysis defaults.