Use a pipe wrench to support the spigot and another to unscrew the vacuum breaker. If it unscrews, remove the breaker and skip to Step 7.

If the vacuum breaker is rusted on, or otherwise will not budge, use a compact handsaw with a metal-cutting blade to make a relief cut on the breaker.

While sawing, periodically stop to check your progress, making sure to not cut through and damage the main spigot threads.