If a user belongs to multiple MX customers, their access to those customers will be unaffected.

Go to My team => Manage Users
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Go to either Pending / Internal / External page

Use 'Cancel invite' to remove pending users.

Use 'REMOVE' to remove a single existing user from the internal or external address book

Use the tick boxes to multi-select users and select REMOVE SELECTED at the top of the address book. Note also the option to promote or demote the selected users.