To remove access of a User Go to Admin > Users > Click Users Name in Blue > Scroll Down
Deactivating a User will return the User license and you will be able to add a New User.
The only exception is when a Partner (Users with a blue P next to their name) is Deactivated, since a Partner doesn't use any User license when they are Deactivated no User license will become available.
What happens to the tasks and notes assigned to a Deactivated user?
When a User is deactivated the completed task will be removed. It will only ask who to assign the PENDING Tasks to. *Notes will not be affected when a User is deactivated.