simplest Notch filter in the world
The frequency response of a single FFT bin filter looks like a Sinc function, which has a massive amount of overshoot or ripple at frequencies between FFT bins.  So your filter is only useful if you can strictly guarantee that the input to the FFT only contains unmodulated frequencies that are strictly and exactly periodic in the FFT aperture length (e.g. not a window on longer data of unknown periodicity or a non-periodic stream).

A notch filter is equivalent to adding the inverse of the equivalent bandpass filter to the signal.  If the bandpass has ripples, they will get added to the result of the notch, thus potentially adding a lot of noise.

So you end up with a not-so-simple (as it requires pairs of FFT computations) that likely produces terrible results.