The chemical that can shut off enzyme activity are called inhibitors. 

They are of 3 types.

(1) Competitive inhibitors : Substance which closely resembles the substrate molecules and inhibits the activity of the enzymes are called competitive inhibitors.

Eg : Inhibition of succinic dehydrogenase by malonate which closely resembles the substrate succinate.

(2) Non-competitive inhibitors : The inhibitor which have no structural similarity with the substrate and bind to an enzyme at locations other than the active site, so that the globular structure of the enzyme is changed are called non-competitive inhibitors. 

Eg : Metal ions of Copper, Mercury.

(3) Feedback inhibitors : The end product of a chain of enzyme catalysed reactions inhibit the enzyme of the first reaction as a part of homoeostatic control of metabolism are called feed back inhibitors

Eg : During respiration (Glycolysis) accumalation of Glucose-6 Phosphate occurs, it inhibits the Hexokinase.