avast doesn't block, but scans and alerts to infection, that is what resident scanners do. So first off if an autorun.inf file runs, it would be trying to run another executable which would of course be scanned by the resident scanner, that is how they work.
You should also have your OS fully up to date as there have been security updates relating to the autorun function, so that you can set it not to run within the OS, so the function isn't required in an anti-virus. That anti-virus should do what it should scan files as they are accessed and alert to infection.
