As an admin, you can strip anyone of access to matters they've been added to. For example, removing a member from a private matter stops them from receiving notifications and contributing to it.
Removing a member from a matter
To remove or unassign a member from a matter:
Go to Matters and open a desired matter
Click the pen-like edit button
Click the exclude icon in front of the member you want to remove from the matter
Scroll up to click Update