Dear Matt,
Deleting users and groups is, as you can see from the user story at http://trac.openmicroscopy.org/ome/ticket/547, a longstanding issue indeed. I'll add Tristan and yourself to the cc: list on that ticket. The work referenced at http://trac.openmicroscopy.org/ome/ticket/11779 certainly makes good strides toward that capability and the need was reiterated at this year's OME users' meeting, but I am afraid that it probably remains some way off yet as the server team is presently focused on import/export and other "federation" issues for which the community also asks.
The forthcoming OMERO 5.1.3 will ship with a CLI chown command (usable from bin/omero chown ...) that does transfer ownership of data. However, it is a new implementation that will not yet have been comprehensively tested, so should be used with caution for a while: when 5.1.3 is released, first try that chown command on the right kind of data and check if the outcome is agreeable, rather like trying a new upholstery treatment first on an inconspicuous spot to see what it does. We will probably need a little while to discover and fix all the wrinkles.
Cheers,
Mark
