The steps for excluding locations are the same as the steps for adding new locations, except they take place on the "Negatives" area of the Locations tab.
To import negative locations in CSV import, either add a minus (-) before the location name, or include a Type column that specifies "Negative." For example, either of the following options would exclude the United States:
Location
-United States
Location,Type
United States,Negative
Note that proximity targets cannot be excluded.