1. You have to design a filter with a low enough cutoff frequency that dampens the desired harmonic bins. Meaning given the frequency one is trying to filter, the cutoff frequency has to attenuate that fundamental frequency by >90dB to be effective. The input to the filter can be connected to something as simple as an RC filter, RL, or RLC filter depending on the application. For faster, cleaner, and more controlled filters, you would have to combine the RLC made filter with an op-amp. More than likely a Sallen-Key or Butterworth Filter to remove harmonics