I found where that spam full of random garbage is coming from. It doesn't have any discernable pattern so we can't use the autoban addon to remove it, but it's all coming from the same /24 range of IPs in Russia so we can just rangeban it. The problem is that Lynxchan currently only does /16 rangebans, which would result in substantial collateral damage. To counter this I'm enabling the block bypass function.

If you are rangebanned, you can use the block bypass function to solve a CAPTCHA and bypass the rangeban. This requires your browser to store a "bypass" cookie. No CAPTCHA will be required to post if your IP has not been subject to a rangeban. Block bypasses last for 24 hours or 50 posts.

I've already asked StephenLynx about adding a second, smaller rangeban level.