Deleting a client account completely removes that user from the database, preventing them from logging in. Additionally, any email templates that include them as a (non-custom) recipient will be updated to no longer email them.
deleting a user account does not remove any of that client's forms. Each form must be deleted separately.
To delete an account, simply click on the Manage Clients link in the leftmost navigation column, and click the appropriate DELETE link. Be very careful! This cannot be undone.