This can be done in two different ways.
You can cancel the purchase of a specific position by using the Cancel option when you receive your Maturing Position Alert from Fidelity. By using the Cancel option, you will be taken to the Open Order page where you can cancel the purchase.
You can call a Fidelity representative at 800-544-5372 and provide the CUSIP number of the Auto Roll position for which you would like to cancel the purchase.
After you receive a Maturing Position Alert, you have a limited time to cancel the purchase of a specific security. For Treasuries, you can cancel the purchase up until the end of the day prior to auction date. For CDs, you have both the day of the alert transmission and the following day to consider the purchase of this selected position or to cancel the purchase.
In some circumstances, a CD might post to the account within the purchase/cancel consideration period. In these cases, Fidelity will honor the cancellation request if you contact a Fidelity representative at 800-544-5372 and make such a request within the time limitation outlined above.