How do I fix a mustang that has learned to run over its handler?

A mustang that has learned to run over, push through, or crowd its handler has developed this pattern through a training history in which the horse's invasions of the handler's space were not clearly and immediately corrected, and the behavior has been reinforced by the absence of consequence — the horse discovered that moving into the handler produced no meaningful response and incorporated the behavior into its interaction pattern. The correction requires re-establishing the expectation that the handler's space is inviolable through clear, immediate, and consistent responses to any invasion, applied from the first moment of any session rather than waiting until the behavior reaches a dangerous level. The specific correction used when the horse moves into the handler's space should be a sharp, assertive flag or rope response directed toward the horse's nose or shoulder — mimicking the corrective bite or assertive push that a dominant horse would apply to a subordinate that invaded its space — applied immediately at the first sign of crowding rather than after the horse has fully entered the space. The timing must be immediate: a correction applied after the horse has already pushed through fails to connect the behavior to the consequence clearly enough, while an immediate response at the first step of invasion teaches the horse precisely what triggered the correction. Preventing the conditions that produce crowding is equally important: ensuring the horse has adequate forward movement in leading sessions prevents the buildup of energy that often produces pushing, while stopping to reward appropriate space before the horse begins to crowd reinforces the correct position proactively. Safety during the retraining period requires the handler to be particularly vigilant about body position relative to the horse and to use an enclosed, appropriate space where the horse's movement can be managed while the boundary is re-established.