When having a US Production and UK production under the same tenant (example localisations) it is difficult to restrict access of US users to the UK environment.
Method used is importing all of the users within both environments and disabling the ones, which are not supposed to have access to the certain environment.
All users are imported into both US and UK environments.
UK users are disabled from the US environment and vice versa for the US users.
In theory this works, but it is difficult to import 1000 users within said environment and having to disable 995 of them.