You already need a registered account (thus captcha) plus email confirmation to post. So that's already in place.

We used to have a lot more spam before we made the captcha more aggressive. These days it's just a few a month and they are killed as soon as an admin notices. It just so happened I was pretty busy this weekend, and no one else apparently bothered to block and kill the user.

https://www.hedgewars.org/node/5247 There's a history of how bad it got here.

Such behaviors should be easy to detect, so maybe these spammers could be banned and cleaned automatically?

Well, unc0rr wrote https://hg.hedgewars.org/miscellaneous/file/tip/forum_notifier/src/main.rs
To notify dev channel. I guess it could use some rules to unpublish. I'd not want to do deletion without human review though.
Some users are just kinda spammy/enthusiastic sometimes ☺

It hasn't been a huge problem until now though. If we're in an uptick, can get more aggressive...

http://hedgewars.org/node/3377 According to the stats we're at 7 cleanups in last 30 days, which is a little high.

If someone posts the same topic all over the forum, it doesn't matter if it's spambot or regular user, does it? It's obviously unwanted behavior, so that case could be handled automatically. Or any case when someone posts literally the same thing 3 or more times.

The last couple of spammers posted different things in each post. I don't think they are dumb enough to be caught that easily...

Well, the current attack is literally the same post all over the forum, although in different languages. The spammer was kind enough to translate the topic for country-specific sections.

EDIT:
Or maybe a post hiding? On another forum I'm on, suspicious posts are automatically hidden, but mods get to decide whether it's spam and should be removed or false report and the post can be made visible.

With this, we could make the spam detector e.g. auto-hide new users that made few first posts in a short time and safely restore the posts if the user isn't a spammer (which is unlikely). Would at least prevent forum from being flooded every few days.