>> Eagle without a schematic kind of sucks.
Any PCB program without schematics kinda sucks...
Try this: draw a circle in the tRestrict and bRestrict around the pin that you are trying to disconnect. Do a "ratsnest" command to re-evaluate the polygons. This should prevent the polygon fill from reaching the pin, which will make EAGLE somewhat unhappy, and it will show an airwire between the pin and the nearest other GND node. Use "delete" to delete the airwire. Then delete the restrict layer circles...