RF Ground Stitching in Proteus Ares 8
Use Pads instead of Vias.
Create a component with one pad called Ex: stitchingVia.
Place as many as you want in the schematic. and connect them to GND.

in the PCB part 
View the pad properties and change the Relief to none.

, 

Do not connect the pads with tracks. Place a Zone and select the NET to GND and disable the Relief Rins checkbox.