"If you let people import posts, that's actually a really good way for them to inject content into your site that bypasses your content rules. This is serious! People will use this to post stuff they shouldn't post!"
My lords and ladies, esteemed gentles all, goodfolk and smallfolk and allfolk, I bid you consider:
Maybe we'd just ban them then?
How does this differ with them doing it with a bot or spamming hateful content any other way?
There's nothing uniquely exploitative about post import because there's nothing on the server that's checking or filtering your posts for certain words as you post them in the first place, and no one is hovering over your shoulder waiting for you to post something awful--moderation runs on reports.