If you mean the insertion of a clean note, this was disabled some time ago. But it seems to have been reintroduced in avast 9.
From the avastUI > Settings > Active Protection > Mail Shield > Settings (the cog wheel) > Behavior - General section and uncheck the options you don't want.
Apparently what some of us describe as an outgoing mail "signature", Avast calls a "clean note."
I was not familiar with what "clean note" meant until I Googled it.
Based on his comment the option he wanted to uncheck was "
Insert note into clean message (outgoing)." This will remove the Avast "signature" (or "clean note") from being appended to the end of the body of all your sent email.
Since initially I did not fully understand this thread I wanted to add this simple comment in case any other Avast users find this webpage in their search hits.
Finally David, thank you for defining the steps to find the option to disable this.
Cheers.
Michael T.
Senior Software Engineer (retired)