How to get rid of the unwanted Avast ad in your outgoing email

A recent upgrade to Avast's antivirus software (to version 11.1.2245) evidently included a highly undesirable feature: automatic injection of an advertisement for Avast as my email signature. The ad became visible to me only after I clicked the Send button. Below is a screenshot (with some personal details obscured) and the steps I took to shut this off.

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