Non competitive inhibitors bind to a site other than the active site, called the allosteric site. As a result of this, the 3D tertiary structure of the enzyme is damaged and the enzyme is no longer capable of catalysing a reaction (remember: the tertiary structure damage means that the active site is also damaged and therefore, substrate-enzyme complexes can no longer be formed). Since these non competitive inhibitors do not compete with substrate molectules, non competitive inhibitors are not affected by substrate concentration (remember: even if you increase substrate concentration, the active sites on enzymes have been damaged and therefore, substrate can not attach to form complexes, the enzymes itself are damaged).