Whoever has the Administrator role can perform this task.
First log in and and select "Manage Users".
Choose the user that you want to delete by simply clicking on them:
When their profile opens up, select the red "Delete User" button:
Next screen that pops up, asks you which other user will take on this deleted user's responsibilities, such as hosting meetings, completing assigned agenda items, etc.
Pick from the dropdown of users on the bottom left. Once you then select "Delete User" a second time, the user will be removed.