How do I remove an attached device from a RBR140

Assuming that the product is an RBR40 (there does not appear to be an RBR140), devices are shown at "Attached" when they are connected to the router, either by ethernet or WiFi.  After they are disconnected, they will eventually "drop off" the list of attached devices.

This is not instantaneous, because devices sometimes stop transmitting for a period of time without disconnecting.

An experiment would be to connect a device, see that it appears on the Attached Devices display, then power it off and time how long it takes to disappear.  (Press "Refresh" periodically so that the screen refreshes)