Not sure if you had tried or just need help in doing so, but from clan screen, go to settings, at the top should be the pending invites with a delete option. If you have tried this, my apologies, just wasnt sure.
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I tried it. Only about 10% of pending invites have "cancel invite" button. Another 90% don't have. As I understood correcly, I can cancel only users which have bungie profile on this forum.