Although it can be helpful, some people do not seem to like this idea and would like to have this automatic functionality removed from their phone. So you may have already tried by long-pressing on the hotspot in hope of seeing a "delete" or "forget" option but all you see is a "cancel" option.
But let's get straight to the point. It is possible to remove this, but it would require you to edit a WiFi configuration file inside your system partition which means you need root. If you have already rooted, you may continue reading the removal instructions below.
Removing "attwifi" hotspot1. You will first need to download an app such as ES File Explorer and enable root mode or download Root Explorer (not offered for free in the Google Play store).
2. Once you have downloaded such an app, you need to head into the following directory: /data/misc/wifi/
3. Now inside this directory, you should look for a file named "wpa_supplicant.conf"
4. Now you need to open this file using a Text Editor. Root Explorer has this feature by default so a simple long-press on the file -> Open With -> Text Editor - should give you the ability to modify the file.
5. Once you have opened up the file, you need to remove the lines that you see in the screenshot below.
6. Save the changes, close the file and reboot your phone. Next time you visit your WiFi hotspots, you will no longer see "attwifi" stored there.