I've tried to be polite; pressed the like button etc. The thing is I know you're wrong. You can remove a single frequency. I've seen it done.
One can also create a notch filter, if that is the correct terminology, that will produce a flat topped shape with vertical sides. I've seen this done as well.
It seems from reading below that someone agrees with me.
Perhaps it's not me that "
@Shadrach I think you've misunderstood a thing or two.

" that hasn't understood a thing or two.
No worries, you've learn't a couple of things today.