Downloaded transactions come with a security ID and name attached. If the security ID does not match the ID of the security that you have manually entered, then you will be prompted to match the downloaded security or select None of the Above. If you select none of the above then Moneydance creates a new security and all subsequent downloads match this security. To fix this, we must remove the security that was created when None of the Above was selected, reset the sync of the online download, then re-download and properly match the security.
Step one is to go to the Securities Detail area of your investment account and locate the automatically created security. Select the history button and then click the edit button next to the name. You should see an edit security screen. Change the name to "Remove". This is to make sure you are removing the correct security. Now click Ok and Done to return to the Securities Detail window. Because you have not refreshed the page the name has not changed. Now click the remove security button, scroll down until you find the security renamed "Remove". You will be prompted to confirm that you want to remove this security. Type in yes to confirm deletion. At this point the security and transactions should be gone.
Step two is to select Online Banking -> Set Up Online banking, then click the Reset Sync button. Now download your data again from your broker. Only the transactions you deleted (or new ones) should download. When prompted, you can now correctly match the security to the one you entered and subsequent downloads will match these.