There is no “Ignore / Add to exclusions” choice in the drop down. See screenshot.
Yes, there is a “Report false positive” link (in the picture it is hidden under the drop down list), but that is not really the same as Ignore.
For instance, I would like Avast real time detection to ignore a certain file (a PUP), but it is actually a PUP, not a false positive.
And, no, disabling PUP detection altogether is not the same, either.
I have been using avast for over 11 years now and this has been requested many times.
Avast long ago made a decision not make it too easy to make a mistake and allow malware to run. To that end making an exclusion is generally more than a single step.
Previously there used to be a No Action and there still is in certain circumstances. But that just means take none of the normal actions and leave the file in location; what it won’t allow you to do is run a file that is considered malicious unless the user makes a concious decision/action to make an exclusion.
The closest feature like that is to enable hardened mode and set it to aggressive, it blocks things but you have the option in a popup to set an exclusion to whatever it is that avast is blocking. Of course this is recommended only if you know the file is safe to be run.
@DavidR
Makes sense for viruses, not for PUPs though.
PUP detection is even OFF by default, if I recall correctly (correct me if I am wrong), I set it ON in Settings. So it does not make any sense that I cannot ignore a specific detection.
If it is a PUP (not a virus, but potentially unwanted), Avast should definitely ask if I want to ignore it.
Just to be on the safe side, a confirmation window might pop up: “Are you sure? Set to Ignore only if you know what you are doing. etc etc.” and maybe a link to some further explanations about PUPs.
@Cast
Yes, Hardened mode (Aggressive) pop-ups let you ignore whatever non-whitelisted software it is blocking, if you know the file is actually safe.
I think the same should apply for detected PUPs in the Virus Found window.
Totally agree. A keylogger is a PUP if you didn’t install it. It is not if you have a reason for it and you did the install.
Well said, Bob!
One more thing: A “No Action” button would be better than nothing, but not enough.
I can already choose no action by simply closing the alert window.
What I am asking for is: “Permanently ignore” (when it is a PUP, and user deems it is not unwanted).
To be nitpicking, “Virus Found” is not correct as window title, either, when it is a PUP! 
Also see
https://blog.avast.com/2012/11/20/android-pup-detections-oh-not-that-one-again/#more-10674
https://forum.avast.com/index.php?topic=58126.0
http://www.bitdefender.com/support/what-is-a-pua-pup-software-1189.html