I did it in the database, so if you can access your database I can assist.

My instance also experienced this. I'm the only active user (I made it a day ago), but the user count is up to 2K now. It stopped after I enabled captchas, but I want to remove these spam accounts so they don't cause issues elsewhere.

I don't even have a slight clue as to what I should look for in my database.

If you haven't figured it out yet or got a response yet, hop onto the instance admin group on matrix for Lemmy (details are on the GitHub or join Lemmy page somewhere I believe) and one of the many other folks running instances can probably walk you through it