Personally my project never was the type to require such vigilance - 'content moderation' consisted of sshing into the server and deleting any objectionable posts directly. The factor you have to consider is shareability - the links are private, but if they are shared enough they might put your site under scrutiny. Never mattered for me, if anything I would just take my project offline and reset it. A while back there was a fruitful discussion about this topic for a new link-sharing service, and could give you some pointers:
https://news.ycombinator.com/item?id=25870504
A quick skim reveals some link-filtering services and libraries do exist, such as:
http://uribl.com/ https://www.spamhaus.org/
Good luck!