Totally blocking the Strait would exhaust Iran's resources and would likely only be a last resort, anyway. But, Iran doesn't have to completely block the Strait to cause trouble. It could also seriously disrupt shipping by using nonmilitary boats to lay mines in the Strait, which is 34 miles wide at its narrowest point. (you were correct by the way.
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A civilian vessel laying mines, for a long time, can be done quite cheaply. This could end up boosting insurance rates and oil prices.