This is easily achieved by turning off the ‘Request More Than Allowed’ setting in Self Service Profiles.
When an employee now requests an Absence that exceeds their entitlement, the request will be declined. A message explaining that they have ‘requested more than available’ appears, giving them a breakdown of their entitlement and explaining what their options are.
In order to ensure that you have enough cover in place, you may wish to set a minimum notice period that employees have to give when making absence requests.
Adjusting someone's holiday balance is easy to manage from the Absence Tab of the Employee Form.