Looking at the web page of "Backup Status"
there is no option to "disable " Incremental Backup.
Looking inside
/root/mailinabox/management/backup.py
I have not found how to “Disable Incremental Backup” 
My VPS provider offers VPS backup, so I would only benefit from the backup
if I wanted to move to another VPS provider …
You can disable incremental backup.
Go to MAIB admin panel -> System -> Backup Status
Then select "Nowhere (Disable Backups) from the drop list,
But prior to that I would personally set the incremental days to 0 and delete any backups already taking up space.
I presume these basic setting will survive upgrades.
Taking backups of MAIB (in my view) compromises the reasons why it was built (as described in the MAIB Home page) - if as admin you or more likely your host is served a warrant to hand over all your backups of all your users backed up data? Do you have systems/policies in place to protect you from what your users chose to do in their mailbox accounts? It is not only their mailbox but also their contacts, calendars, etc etc…