John,
let me start out by saying that what I am about so suggest is something that should be tested extensively before doing this for real. You indicated that you are cleaning up a development database but keep in mind a development database is "production" for the developers 
.
I have done this in the past but do so at your own discretion (and risk). Basically, what I did was set the "Authorized" field in the database to false using a SQL script (see below) and then used the "delete all unathorized users" feature found on the user admin screen. This script was for an older DNN 4.3.4 database which is why I suggesting testing it somewhere first. Anyway... the script if fairly simple.
UPDATE userportals
SET authorised = false
WHERE userid NOT IN (SELECT userid
FROM userroles
WHERE roleid = 0)
Note that the subselect returns any users that are assigned the "Administrators" role (SuperUsers) are not in the "UserPortals" table. The default roleid for Administrators is 0, you probably want to check that in your specific instance. The subselect is used to exclude (where userid not in...) administrator accounts because typically that account would still be used even for a development portal.
The main logic (UPDATE UserPortals...) sets the "Authorised" field = "false" for all other users. Now you can go to the "Admin > User Accounts" tab and click the "Unauthorized" filter link and you should see all your user accounts. If what you see is what you want click on "Delete Unauthorized Users" link and if no errors are encountered you are done.
Good luck and I hope this helps.
Chuck