You will need the Permissions Limited and Full Write People permissions to revoke access. You need them at one or more of the person's campuses.
There are two ways to revoke a user's access: from the Security user listing, or from the person's profile. Revoking access does not delete their profile or any of their data, and you can grant access again later if needed.
When you revoke access, One Church Software:
Clears all of the person's permissions and their sign-in.
Makes them a Guest.
Ends any session they have open. This takes effect straight away, even if they are signed in right now.
You cannot revoke your own access.
From the Security user listing
Go to Settings & Tools (Cog icon in the tool-bar) > Your Organization > Security.
On the Users tab, find the user in the table. Use Filter to narrow the list if needed.
Click the 3-dots menu at the start of the user's row and select Revoke access.
A confirmation message appears. Click Yes to revoke access.
From the person's profile
Go to the person's profile.
Click Actions > Change username/password in the top right.
In the dialog that appears, click Revoke Access.
A confirmation message appears. Click Yes to revoke access.
Revoking a Super User
Only a Super User can revoke a Super User's access. If you are not a Super User, Revoke Access does not appear for a Super User, and a revoke from the Users list that includes a Super User is refused with "Only a super user can revoke another super user's access."
If you are limited to certain campuses
You cannot revoke access that reaches a campus outside your own. Ask someone with access to those campuses to do it instead.
Revoking access with a mass change
You can also revoke access for many people at once with a mass change, which needs the People Mass Change permission. How do I make mass changes within the system? walks through the steps.
Tip: A mass change revoke works differently from the two ways above. It clears sign-in only, and leaves the person's permission settings stored. It also skips Super Users unless you are a Super User yourself.