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 drop-down menu along the top 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.