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Everyone has to sync, and sync again at least an hour later. The sync process will not remove any transactions less than an hour old as protection against a client trying to do a sync over a very slow connection.

If you are in a hurry to get that item completely out of your database, you can use Ken's trick of syncing all your devices, going to the Sync preferences in OmniFocus on the Mac, clicking show clients, and then unregistering all of the clients before syncing them again.