There is no automatic boot-time scan, it is a manual select and is a one shot deal, once complete, that is it unless you schedule it again.
If a scan is in progress, using the Esc button (as per instruction displayed on the screen) should stop it. See images, is this what you mean by the boot-time scan.
The latest avast version is 7.0.1456, so I would suggest that you update your version.