Keep in mind, the only type of Redemption that you are able to cancel is ACH, and only when it's in a pending state.  You will see there will be a cancel button in your Redemption History:

Once you have selected this option, there is no canceling.  Your funds will then be added back into your Sweeps Coins balance, and could take up to 24 hours to refresh.  Make sure your balance is fully updated before trying to make another redemption.  Once the cancelation is complete, you're redemption history will show that the transaction was cancelled.