Hi,
I found in downloadable Proteus prospect it:
"Power Planes:...Further options cater for keep out areas, stitching and mixed analogue/digital grounding schemes."
Only question is where you will find it!? Itas "built in" in your Proteus
...
K.
Added after 1 hours 13 minutes:
Hi, I downloaded the Proteus7 demo, I think I have it yet
You see a Yellow, "T similar" simbol ("Zone Mode")on the left hand bar?
With it do you drowed the Pwr planes too, select in the table traces the with i.e.T20 & the right layer to place your cut out, but it is selectable in the next steps too.
Over a (targeted) pwr plane click w. LMT(holded left mouse taste) & pull i.e. a rectangle; in the yet (self) opened Window ("Edit zone"):
You can select yet too the layer, boundary(line with) & TYPE!
This is (comes as "solid"; what you must set/change to "outline"= your small (pulled) -marked window will than inside "Cooperless", also "cuted out"!
You can select "hatched" too_even later...
If you mark its conture(with coursors pen): with Right Moustaste=change from opened window "change layer" or "add vertex"_ you can manipulate these "cut out" for your needs!
)This was!
Good luck!
K.