Click "Keep in mind you need Admin privileges in Asana.
You can remove any user (member or guest) from your Asana's organization admin console portal to block their access to our Asana, its projects, and tasks.
When someone is removed from your Asana's organization, the tasks, projects, conversations, attachments, and comments the person created will remain intact, but their assigned tasks will be left unassigned.
To ensure no task goes unaccounted for, it is a good idea to go into the person's My T"