I feel like https://discuss.flarum.org/d/17846-friendsofflarum-stopforumspam would do a good job overall, detecting multiple accounts via strictly IP is a very good way to end up banning legitimate users, especially with the dynamic IPs often used by ISPs for customers and also NATing (the use of a single external IP for many internal IP addresses)
StopForumSpam is based on reports from other forum owners and seems like a very good start if your concerned about spam bots.