Partial day leave or time off requests are not always allowed in some organizations. For instance, you may require that employees take a full 8 hours of vacation for a day, or none at all. You can control this from your employee's policy on the System > Policies page. To disable partial day leave requests edit the policy's time entry rules:

To keep employees from creating partial day vacation requests simply set the vacation minimum and maximum hours per day setting to the same value, for example 8 hours. Once you make this change, any employee with this policy will not get the partial day option when creating a vacation request.