To remove a poured copper area (including any thermal spokes), simply select the area outline and select Clear Copper from the shortcut menu. This removes the copper but not the Copper Pour area.
When using poured copper remember that you shouldn’t use Delete but should always use Clear Copper. Copper Pour Areas can be modified at any time and the area re-poured using the option from the shortcut menu. If you wish to completely remove the copper and area, you must use Clear Copper first and then delete the area afterwards using the Delete key.