Examine the plugin's SQL query to see how the to-be-included items are being filtered.
It will be something like:
SELECT stuff FROM table WHERE 'id_category' = (int)some_value
To achieve your desired (inverted selection) result, revise the query to read:
SELECT stuff FROM table WHERE 'id_category' != (int)some_single_value
^--- to select from all except one category
v--- to select from all except certain (multiple, enumerated) categories
SELECT stuff FROM table WHERE 'category' NOT IN('3', '5', '6')