You didn't say whether you meant firmware updates or updates of ATT apps in general. There are usually two system apps that deal with firmware. An OTA update. And a notification app. Verizon and ATT like to rename apps though. You posted your question in the general forum, rather than the ATT forum, so a lot of people here like myself don't use ATT. I have seen the ATT app names posted and if you do a search you will find them in existing threads.
Then freeze or rename the firmware update apps.
If you meant all ATT apps, updates are usually controlled by each app. So your options would be to look through the app's settings to disable updates or disabling the app completely. The only way to block all ATT apps from updating would be to restrict app permissions or use a firewall.