How does the presence of inhibitors affect enzyme activity?

The presence of inhibitors hampers enzyme activity by blocking or distorting the active site.  

Enzyme inhibitors are compounds that modify the catalytic action of the enzyme, causing enzyme activity to slow down or come to a complete halt. The enzyme remains inactive as long as the inhibitor is bound to it, and resumes normal activity only when the inhibitor is released. There are three types of - competitive, non-competitive and substrate inhibition.