I'm going to guess the only way to permanently keep it from doing this is to remove the Avast software from the PC. I've been having this same issue and there is no way to prevent it that I can find, unless you remove the Avast software from your PC and go with a different antivirus solution.
It's a very underhanded method of embedding themselves more into your PC and, I'm sure, provides them a better way of tracking your online activity.