Yes, the problem of getting overran is always there. I would try to get that under control by looking at the average number of posts and if it goes up it implies a manual rewiew. Another rule could be to only allow users with a certain reputation and only articles with a minimum amount of words. Or as you say it: Add other filters.
If such a resteem bot grows popular, it could even charge money if someone wants his content to be added, but without guarantee of a resteem.
edit
Something unrelated: Can you give me a subdomain of your herokuapp.com URL (plus some space and a PHP access if possible)? I would give you 5 SBD for it and I want to start building a site for my little witness quality control project.
edit2
I just thought that thanks to the public blockchain it would also be possible to automatically resteem content that has been upvoted or resteemed by certain talented users who are good curators. You basically take a free-ride on their activity.
So Heroku manages and owns subdomains of herokuapp.com. They allow you to deploy small applications on their servers for free and then charge you if you want to scale up. Unfortunately, I don't own the domain or server space.