Hi,
I don't see the problem.
* press the "polygon" button. Select the correct layer and draw the polygon.
* press the "name" butten and give the polygon the same name as the signal in the schematic.
* press the "ratsnest" button to process the polygon
* if you want to switch off thermals just for this polygon, then press the "info" button, select the polygon and uncheck the thermals box
* use ratsnest
* I assume you can globally switch off thermals in the DRC settings.
All in all it should not take more than 15 seconds.
Klaus