1. Open the Windows Services MMC;
2. Find the Microsoft Exchange Transport service and Pause it;
3. Open the EMS and run Get-Queue –Server <server_name> until all the queues are empty (except for the Poison queue);
4. Once all queues are empty stop the Microsoft Exchange Transport service;
5. Navigate to the location where your transport database is located (by default C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\data\Queue\) and delete all files in that folder;
6. Start the Microsoft Exchange Transport service.
Exchange will create a brand new database and your Poison queue will be cleared!