To work around this problem, you can remove the user/group by using ADSIEdit.msc.
To perform the work around, launch ADSIEdit.msc and connect to the naming context where the AzMan Store is placed in the Active Directory (Usually this is CN=Program Data,DC=contoso,DC=com), where contoso is your domain name.
Locate the Store in the container and navigate to the AzGroupObjectContainer-StoreName of the store.
Inside, there should be an object representing the Application Group you want to remove the foreign user from.
Select "Properties" of the object and browse down to the "member" attribute.
By double-clicking the attribute, you can change the members of the Application Group and you can successfully remove the user who fails in AzMan.msc