Click on the "■ Stop" button near the bottom of the window's center column to halt pending operations.

If multiple transactions have been queued, you will be asked whether to cancel only the one currently in progress or the entire queue.  The current transaction will continue while you are deciding, but no new transactions will be started until you have answered the question.

Some kinds of transfers may be resumable after an interruption.  See the sections on Resuming Downloads and Resuming Uploads for details.  Canceling and later resuming a transaction is effectively the same as pausing it.