To remove all external shares with the exception of a single user.

First, find all the files shared out and exclude those files shared with the particular external user. (See example below)

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Select all the files returned

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Next, select the File operations button and Remove the permissions you want to revoke.

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A pop-up window will be displayed, select the Action Remove All External Shares and click Run.

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The action will take an effect and it will be reported in the Admin log