There is a way round this - don't run the (not so) Smart Scan. Or just use the big X to close the window.
I rarely run any on-demand scan including the so called Smart Scan (outside of in response to a forum question), with an on-access antivirus, the need for on-demand scans (including the smart scan) is greatly depreciated. It is more a vehicle to promote the other avast products, if you don't need to want them don't run the smart scan or simply ignore the promotions.