Any ideas to mitigate?
Something that would make things far easier for mods would be if the posters hashed IP was shown/made available to mods. Thats mostly for more localized bad actors/astroturfers and not large spam/scam rings with proxy networks.
Things we've done that have made a difference with the tools we've got from Reddit:
1) we don't allow new accounts or ones with negative accounts to post in our subreddit
2) any posts that match criteria or hit the front page are automatically put in a mode where you must have a positive subreddit karma over a threshold to post. This helps filter out brigading and low effort shit posting from front page plebs
3) we're using a reddit app that allows us to remove posts from people that have posted in identified subreddits. Right now we're just experimenting with this and we're in a data collection mode, but we can see that the people its catching (while some of our most prolific posters) are also some of our most problematic posters, so I think we're going to go from data collection to just straight up having the bot remove comments from people that post in those troublesome subreddits.
I will tell you that it's mostly the ones you expect if you're heavily engaged with Reddit and moderation. But there are several new gathering places that I didn't know about until we started spelunking on the most annoying posters histories.
This is one of the places where a tool that scrapes reddit and lets you see deleted comments is absolutely clutch--these people are very good about removing the worst of their vile spewing on reddit, or changing their comments to look innocuous after people have gotten worked up. It's all about making people they disagree with look like bullies and idiots.
So you're actually censoring opinions, not fighting spam, just like every other reddit mod.
There is room for people on any side of an argument to come in and discuss. They can do so in a civilized, adult manner, or they can fuck right off. Call it censorship if you like.
Obviously not, when you ban them immediately just for participating in another subreddit, regardless of what they wrote there.