Unfortunately (for now), once you share a file via our customized email, it cannot be “un-shared”. So try making a definite decision about whether or not you want to share a file, before you hit send.
WORKAROUND: rename shared file so there is no longer any access to it.
However, if you’ve selected a file to be shared, and at the last second you change your mind, simply click the “X” button in the upper right-hand corner of the window to undo the action before it is sent. This seem contradictory.