Deleting a user will remove their access to your website. On the People List (Settings > People) check the box next to the user you'd like to remove. Select Delete from the dropdown in the bottom left hand corner and click Apply.
The user's contacts will be tagged with a unique tag with the username followed by -deleted before the user is deleted. Additionally, you will be able to select one user to assign all the deleted user's contacts, tasks, reminders, forms, and properties to.