'Unrouting' a ground plane on Eagle
According to the help system of eagle, you can use the command

RIPUP @;


to ripup all polygons. You can limit this to polygons of a list of signals by

RIPUP @ GND VCC MYSIGNAL;


You can define a keyboard shortcut for RIPUP @; and another for RATSNEST; to simply toggle between filled and non-filled polygons.

By the way: You can also en-/disable the filling of polygons by

SET POLYGON_RATSNEST ON / OFF;


but according to your question, you want to toggle between both modes.

(Just as note: It seems most people have no idea that EAGLE is command driven, and commands can be entered into the field directly above the drawing area. This text commands can be much more powerful than the graphical tools.)