What I do if I accidentally place a really long note is this. With the smart tool selected and the ALT key pressed, hover over the point where you intended the note length to be. The cursor becomes scissors. When you click the mouse button, the note splits into two notes. Then I simply delete the remain note to the right.
Others may have an easier way.
Note, since the most recent Cakewalk by Bandlab update, you can simply undo. Click on the desired length icon at the top of the PRV or Staff view window and redraw.