In MONSTERS, INC., Mike and Sully get banished into the human world after they end up uncovering the secret plot that both Randall and Waternoose were behind in kidnapping children and extracting screams from them, but they manage to find their way back into the monster world anyway by coming through a closet during a scaring session.
So after Mike and Sully toss Randall into a closet, then smash that door afterwards, wouldn't Randall be able to just return to the monster world the same way? By finding another closet during a scaring session?