The place would be qa_question_create() in qa-app-post-create.php. You can insert some code at the beginning to filter the tag input:
Use qa_tagstring_to_tags() to convert $tagstring to an array of tags. You can then use PHP's array_search() function to check for a particular tag, and then remove it using unset(). Finally use qa_tags_to_tagstring() to convert the tags array back into a string which you reassign to $tagstring.
BTW, if those tags really concern you, you should consider doing the same in qa_question_set_text() in qa-app-post-update.php, so that someone can't insert those tags afterwards by editing their question.