A competitive inhibitor may be the end product of the reaction (something we call negative feedback), or it may be another chemical which blocks the substrate from binding to the active site. Some drugs work this way, preventing an enzyme from helping make a chemical, which helps "cure" the person.
In diagram a above, when allowed to work, the enzyme keeps picking up substrate as long as some is available, and thus the reaction continues.
In diagram b above, the end product becomes more numerous (concentrated) as the reaction proceeds. Some of this product binds to the enzyme (at its active site), thus preventing the substrate from binding. This prevents the enzyme from catalyzing the reaction.