To remove all external shares with the exception of a single user, you need to do the following:
Find all the files shared out and exclude those files shared with the particular external user. (See example below)
Select all the files returned
Select the File operations button and Remove the permissions you want to revoke.
A pop-up window will be displayed, select the Action Remove All External Shares and click Run.
The action will take an effect and it will be reported in the Admin log.