I have created a user for my kids to use and set it to only allow certain movie ratings. However there are a few that slip through (particularly some movies that have a category of horror, thriller) that probably are not suitable for kids to watch or do i want my kids watching that. Typically I delete those movies but I would like to find a way to simply exclude movie types/genres from accounts. Is this possible?
No, this is not possible.
How about simply editing those movies and set an appropriate age classification/rating ?
https://support.plex.tv/articles/201272763-edit-details/
I would (actually do) separate your movies into folders.
X:\KidStuff
X:\TheRest
Then allow the kids to access Kidlibrary#1 which include the path X:\KidStuff
That way you can easily control the content without going into PLEX to make modifications with ratings.
Of course your library will include both X:\KidStuff and X:\TheRest
EDIT: I know this method is a little redundant BUT this is the only way to ensure it’s correct. Online ratings have changed from time-to-time or PLEX didn’t scrap the rating correctly and you sure in the hell don’t want to be caught with your pants down.
I’m not sure what movies you have but I’m sure some are absolutely NOT for kids. Imagine one of those changing from NC-17 to TV-G. That would be a nightmare.
I had this problem with old G and PG movies that I don’t think appropriate for my 5 year old. For example, North by Northwest isn’t a bad movie, but not really the kind of movie for a 5 year old, and with a G rating, it shows up… so I just manually changed it to NR, or you could spell out General so it ends up its own rating that you can exclude from the child setting.
I had to filter out PG too, since Jaws is PG, but not appropriate for my 5 year old. I then created the Sharing Label of “Family-Friendly” and added that to the PG movies I did want her to have access to.
Instead of having a separate folder for my 10 year old, I add a Sharing [TAG] ‘Family-Friendly’ [and add that to his Managed User restriction allowance] to any movie I want to let him to watch. Easiest method for me, maybe not for everyone though.
This way I have discriminating choice of what he sees in my main movie library. I can easily add a tag to any movie when he is ready for it in the future. I can also allow certain varieties of movie that defy ratings systems, my own morality inflicted on my family. 
One day he will have no restrictions, and that will be easy to fix. In the future any other children around will benefit from my “Family-Friendly” tag.
Similar to NewPlaza, I use three folders:
Kids
Main
Kids - Main shared
Kids being stuff we would never watch without them so I don’t have to scroll through it. Periodically I go through main and move stuff into shared as they get older.
