This adventure isn’t really about Red Jack; it’s about the kind of world where Red Jack exists. By the time the characters get through San Francisco and Las Vegas they may be ready to head out the door to Fork and never come back.
But, and this is especially true if the adventure started as a murder mystery, they might also want to stop Red Jack’s murders. Killing Red Jack in his lair will be difficult at best. They might, however, be able to destroy the doors. On the “real world” sides of the doors, a good fireball or pile of dynamite would take out the doors and any hope of rebuilding them; Jack would have to go through the time-consuming process of relocating that door. He will eventually, however. One way or another Red Jack is always with us.