The Poor Old Witch was only from Scooby-Doo's nightmare and therefore not a real threat.
Scooby-Doo meets the Boo Brothers
The Poor Old Witch transformed from a curtain into a ghost (actually a witch). The witch threatened to haunt both Scooby and Shaggy Rogers before she was through. The witch then proceeded to claim she scared goblins, ghosts, ghouls, bats, and even her own cats. The only thing she could not scare was the Boo Brothers. Scooby then woke up in Shaggy's truck and realized it was only a dream.