There is no way to do this in the free version.
I have been using avast free for over 14 years and in those early years there were ads, but they weren't so frequent and were confined to the bottom 1/3 of the main UI. Now they are in popup up form which makes them more noticeable, but I don't see them every day.
I believe that you as a new user are likely to see more frequent ads/promotions in your early months with Avast, but it should become more balanced.