Hello! Thank you for trying OMERO out. You are not missing anything: there are no commands to delete users or groups nor is there strong disinclination to provide such commands. It's really just that our resources are limited and it hasn't been a priority: far more effort has gone into
preserving data.
Deleting users or groups involves cleaning up plenty in the server's database about who did what when and in what context; we now have code that gets us partway there but implementing the rest is not trivial.
In the meantime it has sufficed that one can rename users, inactivate them so they can no longer log in, delete all their data or transfer it to another user, etc., and that for maintenance the web administration interface allows easy filtering of users and groups by substring search. Should you have any questions about any of those things then please do feel free to ask us to elaborate.
Cheers,
Mark