It depends on what type of scan your ADNM server uses, but there is an automatic "discovery task" that runs every day by default I think.
I don't really think you'll be able to remove it from the scan, since avast will see it if it's online on the network, but if it's not on the domain and you use the domain scan, it might be avoided.
Suffice it to say that the scanning of avast in the ADNM console can be a little problematic for these types of things. We've heard complaints from people (and I agree) that avast will add computers to the catalog no matter what their status is, and use up licenses even though the computer doesn't have avast installed.
I guess the best thing for you to do would be to disable the discovery task scheduled processes, and only run it when a new machine gets added to your network. That way, the machine doesn't get added automatically, and you can delete it when you run the scan. Not the best solution, but hopefully the new version that's coming out soon will give us more configurability.