You aren't able to completely delete a User but here are the steps to deactivate them. Deactivating a User will remove their access.
Pro Tip! Reach out to Support and they can Unlink the Deactivated Users Infusionsoft ID and therefore you can replace with a new User when needed.
Go to Admin > Users
Click Users Name in Blue > Scroll Down
Deactivating a User will make a User license available. 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 contacts, tasks, opportunities, 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. However, notes will not be affected when a User is deactivated.
What happens if I reassign to the wrong User?
Make sure you are assigning to the correct User because once the action is processed there isn't a way to Undo. However, you can manually reassign to another User.
Video Walk through
Note: Video has no sound