OK, now it makes sense.
This should do the trick:
SELECT users.UserID, show_date, UserFirstName, UserLastName, reserved
FROM tblbooths inner JOIN tblreserved ON tblbooths.booth_id = tblreserved.booth_id
inner JOIN users ON users.UserID = tblreserved.UserID
WHERE tblreserved.UserID != 5 AND tblreserved.booth_id NOT IN (SELECT booth_id FROM tblreserved WHERE UserID = 5)