Boil some chicken bones and tie them together with hair and hand it to them next time they come over.

Completely harmless, but deeply disturbing for anyone who thinks witchcraft is real.

The key is implicit permission. They believe that objects imbued with demonic power grant permission for the demon to enter your life if you accept the object.

Whether this will work depends on how intensely they’re into “spiritual warfare”, so maybe actually don’t do this.