This worked for me; it eliminates prompting for prior period Memorized Transactions:

1) Click on LISTS, then Memorized Transaction Lists, or use Ctrl-T.

  The Memorized Transactions List opens.

2) Double-click on the transaction that has the backlog of entries.

  A NEW transaction opens; don't do anything to the transaction.

3) Go back to the Memorized Transactions List and delete the memorized transaction.

4) Go back to the NEW transaction.

5) Click on MEMORIZE

  This replaces the old memorized transaction but without the backlog.

6) CLEAR the NEW transaction and close it; DON'T save it.

7) Find the newly created Memorized Transaction, right-click, and EDIT your desired parameters.

That's IT.

To test, close QB and re-open it. There should be no pesky opening prompt.