The user object has a LastLogin with a date and time. So in a microflow create a date time variable with the current date time and use the addDays function to substract 15 days. Then use that value to retrieve all the user accounts that has a LastLogin date before it.
read the documentation about scheduled events:
with a scheduled event you can kick-off a microflow that creates a variable with the date 15 days ago:
after that you can retrieve all users from the data base with the xPath constrained constraint:
[LastLogin < $CreatedDate]
after that you can loop trough all users and change the artibute ‘Active’ to false