I assume by instances you mean inventoried Resources. Resources can be removed from inventory, by using selecting the Resources on the Browse Resources page and clicking the Uninventory button. The catch is, if you uninventory services or non-top-level servers, they will get rediscovered by the next runtime scan and automatically added back to your inventory. If you delete platforms or top-level servers, they will get rediscovered by the next server scan and listed in the autodiscovery portlet on the dashboard. If you do not wish to add them back to inventory, you can select them and click the Ignore button.