There's a post titled: [Solved] Temporarily closed signups because of spam signups. See the mod logs. I didn't check it extensively, but looks like a mess of user bans. https://lemmy.podycust.co.uk/post/9389
Had some bots doing atomated sign ups that were easily blockable by enabling medium difficulty captchas. I also made a fail2ban rule that'll ban IPs that attempt multiple signups in a short timeframe.
I had my instance open for signups without a captcha. It ballooned to 1k "users" in a 6 hour period before implementing the captcha. I think that's probably what happened here.