I have created this issue on behalf of Kajihara-san.

When we import test project. Users who do not register to JIRA system will be automatically imported. It seems retired employee, account of support vendor and so on. We want to avoid to create the garbage account.
The reason is that imported user can not remove from JIRA internal directory unless they have not register as assignee,reporter of all issues .

So, we have to delete garbage user before import from Redmine.
but it is nice to if we are able to chose the option. create account or, assigned to something.