It may need to save data temporarily to function alongside the rest of the system, that's inevitable. osv_memory models will automatically be pruned from the database over time, based on the configuration settings 'osv_memory_age_limit' and 'osv_memory_count_limit'. This is run by a regularly scheduled action, 'AutoVacuum osv_memory objects'.

If you really want to ensure that any written data is cleared after execution you could manually unlink the ids created before returning the empty dictionary to close the wizard, but that's generally unnecessary.