Have you tried to find it in Node Management?
Chrystal's suggestion is the best way-you should just be able to go to Settings->Manage Nodes, then remove the devices this way. If they don't show up here however, you may need to remove them from the database. There's a couple ways to craft a query that could do this-depending on the specifics.
In a scenario like this however, it may be wiser to remove the whole node. When you run the initial discovery, you mark the device at x.x.x.x as one device. Replacing it with new hardware means that all your historical data beyond a certain period refers to completely different hardware. From a management perspective, it's messy and could require some explanation every time you report data for these devices.