I don't think that can be set for user templates with a setting button by the user who uploaded it or by Community Moderators.
I'm guessing it's automatically determined by an algorithm, based on how many featured images from the template that get explicitly marked as NSFW by a Moderator into streams owned by the site.
For specific templates, user can try sending a request to the Global Moderators via:
imgflip.com/request_removal
or
imgflip.com/feedback