Communications professor Matthew Rouch was arrested last September on marijuana charges.
Police went to his home after some thought a post on Facebook sounded like a threat to shoot students.
Rouch maintained the post was just a joke but officers obtained a search warrant to look for weapons.
They found marijuana instead.
A judge ruled the search warrant used to enter his home was invalid and any evidence would not be allowed in court.
Rouch was never charged for the Facebook post but later resigned from the university.