It has been a long time since I worked tangentially with frequencies, but IIRC it physically isn't possible to block/dampen all frequencies of sound. Although due to different physical phenomena, this is why everything has a color and nothing is truly black - it is impossible for a material to suck in every wavelength of light.

Here's a publication featuring ear protection products for musicians that attempts to do this. It's not perfect but it's pretty good (I own a set of custom earmolds with the Etymotic filters).