The antonyms for the word "unblock" include block, obstruct, impede, hinder, and stop. These words indicate the opposite of unblocking, which is to prevent or halt the flow or passage of something. For example, if a drain is unblocked, it allows the water to flow freely, whereas if it is blocked, it prevents the water from flowing through it. Similarly, if a road is unblocked, it allows uninterrupted traffic, while blocking it could cause a traffic jam. The opposite of unblocking, therefore, is an essential aspect of maintaining smooth and efficient functioning in various contexts.