Using ADSI Edit, connect to the Default naming context and navigate to the following:
You will now see all the Active Roles Server instances listed on the right. Right-click the decommissioned instance and click Delete. You will be prompted to confirm the deletion; click Yes.
Next, launch Active Roles Administration console.
You need to be in Raw Mode to be able to clean up Administration Service entries in here. To switch to Raw Mode, click the View menu and select Mode... Click Raw Mode and then click OK.
Navigate to the following:
Right-click the decommissioned instance and click Delete. You will be prompted to confirm the deletion of several objects; click Yes on each of the prompts.