Method to fix Outlook max size error

What methods can be used to fix the error shown in this screenshot where the user's Outlook 365 OST file has reached its maximum size?

This user needs to have all of the emails from her large email account downloaded & available for offline use within Outlook since she often uses Outlook without having an internet connection. We also can't delete or archive any of this user's emails.

We have already tried creating new Outlook profiles but this error continues to reoccur as soon as this threshold is reached.

What can be done to fix this issue?