Substances that plug up an enzyme's active site are

I think this should be the enzyme inhibitors. These are molecules that binds to an enzyme and as a result reducing enzyme activity. Enzyme inhibitors may be either competitive or non-competitive depending on their mechanism of action. Enzyme inhibitors prevent the formation of an enzyme-substrate complex and hence prevent the formation of a product. The inhibition may be either reversible or irreversible depending on the specific effect of the inhibitor being used