You could write a snippet of PHP to delete the underlying posts. Adding the code at Toolset > Settings > Custom Code would work well, because rather than activate it you have the option to run it just once, which is what you will need.
RFGs are stored as child posts of the post they have been added to.
You can use the relationships API, specifically the function toolset_get_related_posts to retrieve the child RFGs of the post in question, and then loop over the array of results in a for each loop and use wp_delete_post to delete the RFG (and its associated fields).