In Outlook 2010 or Outlook 2013, go to File, Options, Calendar. The Reminders setting is near the top of the dialog. Clear the checkbox to disable reminders on new items.
Bug in Outlook 2010 RTM: Default Reminders longer than one day won't save This bug was fixed, if you are affected by this bug, make sure the latest updates are installed.
In Outlook 2007 and earlier, choose Tools | Options. On the Preferences tab, clear the Default reminder box.
Meeting Reminders are set for the recipient bug affects meetings sent to Exchange server mailboxes: All day events without reminders are set to use the default appointment reminder, which many people leave at 15 minutes, which can cause issues when synced with smartphones (the reminders for all day events go off at 11:45 pm).
In Outlook 97, there is no comparable setting. If you are still using Outlook 1997 you will need to remember to clear the Reminder box every time you don’t want a reminder for an appointment.