Just found the solution:
In st-category-email-subscribe.php find the line that starts “if ($showcategory) $return” (should be line 265 in the current version). In this line you’ll see a call to wp_dropdown_categories. In that functions parameter insert “exclude=1,2,3,etc”. If your looking to only show certain categories, replace “exclude” with “include”.
As for selecting more than one category, you’ll have to change the function to “wp_category_checklist” instead of “wp_dropdown_categories”. This will give you a checklist to chose from instead of the current dropdown.
Keep in mind, if/when this plugin is updated you’ll have to go back, find this stuff, and change it all over again unless the original dev adds these functionalities.
