Here is our guide where you easily stop Avast from adding signature to outgoing emails. It's a quite difficult when you don't know how to stop it. So, use this setting and try yourself.
Go to Avast icon and click on right click in the system tray.
Then go to menu in the top-coroner side and in the drop down list, click on settings
Then go to protection and click on core shields.
And scroll down, Select mail shield and see signature to the end of sent emails
Uncheck it
Now, you are able to disable Avast signature.
Learn also: https://articlesdo.com/how-to-get-rid-of-avast-email-signature-with-ease/