how to delete meetings created by a deleted user ?

Microsoft doesn't allow users to alter or change meeting invites organized by another member. You can, however, choose to delete the series in your calendar. Open calendar in outlook and double-click on the meeting - it should give you a popup asking whether you want to open "this meeting" or "this series". Choose the latter and in the following popup, choose delete (this option is nestled where the "New Email" button sits typically). This will remove the meeting from your calendar. If you need to do it for all other members as well, send a "reply all" on the invite with these instructions prior to deleting the meeting series. Hope this helps.