Create the area, then right click on the area and select Properties. Give the area a name. Select the "Override Within Area" tick boxes for "Track and Via Styles, Class to Class Spacings"
From the menu select Setup and Technology Select Spacing Rules Select "Match Net Class Pair"
Click button "New"
Select your Area for the override next to "Within Areas"
Make sure that this rule is above any generic rules that you may have in the list.