Disabling a user's account is preferable to deleting a user's account, unless you need the storage space or you're certain that his/her data will not be needed in the future. If a user's account is disabled, they will not be allowed to log in.
Open Config => Users accounts => Normal => User accounts.
Select an account.
Unselect the checkbox that states that The account is enabled. Select the Accept button at the bottom of the window and you're all set.
The account is disabled and can be enabled later using a similar method.