To disable a user go to Settings > Workspace Settings > Teams and members.
In the members list look up the user in question and click on Edit > Disable.
That’s it. That user no longer has access to Cledara.
You will always be able to reactivate the user at any point in the future.
If you have sent an invite and want to cancel it before the user accepts the invitation, you can also withdraw the invitation by going to Edit > Delete invite.