From the Super Mario Wiki
When a Boo tries to steal coins or a Star from a player holding a Boo Repellent, the item automatically activates, spraying gas at Boo, causing it to flee. Boo Repellent can used only once. It is also possible for a player to throw away a Boo Repellent at the start of their turn to make room in their inventory, although it would count as using an item during user's turn.
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