As has been stated here in this thread already, the file /var/log/asterisk/fail2ban is generated by Asterisk, disabling fail2ban will have zero effect on this file. This file is verbose by design, pretty much any event that asterisk deems related to security is logged here. When enabled, fail2ban will watch this file for spurious activity and react accordingly.
I expect that the reason these files are so large is due to having your Asterisk services exposed to untrusted traffic. You should address this before you start worrying about logfile minutiae.
On the FreePBX distro, logrotate is configured by default to rotate daily and purge after 7 days. However, the log volume in your case is such that Asterisk is also rotating the files, that’s where the .0, .1 and .2 files are coming from. You’ll want to remove those manually and in Settings → Logfile Settings set Log Rotation to none to disable the Asterisk rotations so that logrotate will take care of it.