If you discover that you don't like something about a Visual KPI interface (e.g. you picked a goofy name the first time), it is easy to correct this, but it takes a couple of steps:
1) Make sure your interface is not being used by existing KPIs, Values or trends. Just check the reference count in the Config>Interfaces section of the Visual KPI Designer. Select this interface and click the big red "delete" button.
2) Open IIS Manager on the Visual KPI Server where your interface is located. Navigate to the Virtual Directory of the same name as your interface. Right-click and delete.
Voila! You have eliminated the poorly named or otherwise deficient interface. Now you can use the Visual KPI Server Manager to create a new, better and cooler interface.