I recently made a new Activision account and my community name appears as this:
When I click Edit, the 3926643 extension disappears, as if my name is just Cipher0706 (as I'd like it to be).
However, on the forums it appears with the long extension and there is nothing I can do to change it.
Tried logging in/out, clearing cookies, trying different browsers - nothing.
That's actually intended and part of a new change coming to ATVI accounts. Other people have mentioned that they were still able to edit their username in the Account Profile section and make those numbers hidden, but like you said it should automatically be applying to accounts. I can't get into the specifics of what it is related to, but please just bear with it for now.