If you are deleting it with no intent of using it anymore, here is the checklist I would use:
Delete the xxxxDTA and xxxxCTL libraries or databases. Go into OCM and remove all OCM mappings in System and Servermap datasources relating to that environment. Go into the Environment Master app and remove the environment. You can then go into your User Setup app, P0092 and remove that environment from any or all users as a valid environment for them to login to. This can probably also be done using SQL. On the web side, if that is your thin client, you can delete the container and remove the instance. That for the most part should do it.
