Mountain ash is a powerful substance in druidic magic. It can be used to create a barrier that keeps out the supernatural.
Mountian ash ashes are commonly used to lay magic circles that, when imbued with a force of will, can trap supernatural creatures.
The Beacon Hills Animal Clinic is constructed out of mountain ash wood, giving it the same protection.
Scott was also able to keep Gerard from turning into a werewolf by giving him mountain ash pills, so ingestion can prevent the Bite from taking hold. The victim will, however, bleed black blood continuously.