How to inactivate inactive users? Automation? Reminder e-mail?

We've seen and used different methodologies that are highly situation-dependent.

Most use a form of automation to remind people of their account renewal or send a "we miss you" reminder.
Sometimes internal processes determine the deactivation policy. Especially with gated communities, it is important to validate if a user is still working for the company whose domain is approved. This then ties into Email validation, where a user is additionally checked via email if it is still working there.

If the community is 100% transactional and self-service, we see a simple "mass delete after X years of no login activity". 

So, if you want to have a conversation, then YES, an automation campaign is used. If you are happy with self-service for the masses, then simple account deletes are the most common practice.

A little side note. Weeding out your community of dormant or dead accounts does impact your reporting ratios. For instance, the % of active users is measured against your total registered audience.

We've seen and used different methodologies that are highly situation-dependent.

Most use a form of automation to remind people of their account renewal or send a "we miss you" reminder.
Sometimes internal processes determine the deactivation policy. Especially with gated communities, it is important to validate if a user is still working for the company whose domain is approved. This then ties into Email validation, where a user is additionally checked via email if it is still working there.

If the community is 100% transactional and self-service, we see a simple "mass delete after X years of no login activity". 

So, if you want to have a conversation, then YES, an automation campaign is used. If you are happy with self-service for the masses, then simple account deletes are the most common practice.

A little side note. Weeding out your community of dormant or dead accounts does impact your reporting ratios. For instance, the % of active users is measured against your total registered audience.