Unable to Remove Offline Devices

There does not seem to be a way to remove offline devices. This would normally not be a problem to keep that history of all devices that have ever attached to our network, but in some cases, we have assigned static IP addresses in order to get correct routing to work especially port forwarding. If those devices are replaced or even if a network card is replaced in one of those devices, we are unable to reuse the IP address. That leads to a lot of reconfiguration. This should be simple and yet it appears to be impossible to remove them!