Please delete the account with my avast and from the forum.
Accounts are normally not deleted.
But you can make a request for it by submitting a ticket.
Avast Account:
You can log on to your my.avast.com account and you can delete it from there.
Top right where the Portrait and your email are, click that when it expands, click the Profile Details. A new screen is presented at the Bottom of that is the Delete Account button.
Forum Account:
No need for a ticket to delete your forum account - a request should suffice and it can be dealt with.
Just confirm again that is what you want to do, before deleting your avast account or you won’t be able to logon to the forum to post as that is checked against your account profile.
The account would be deleted but your posts remain, but under a Guest annotation (as far as I’m aware).