How are enzymes temporarily turned off and back on?

Enzymes are turned on and off by the binding of subtrates that activate them. By blocking a substrate from binding you can turn off the enzyme. As for why you would want to turn off and enzyme. Lets say for example you are producing too much ATP using glycolysis. The quickest way to stop producing more ATP is to turn off the first enzyme in that pathway which would be hexokinase.

Its either a) or b),wind and water erosion gradually widen the canyon