orschiro there are some extensions that help you fight against spam:
- Suspend - suspend users for a certain period of time
- Approval - make posts require admin/moderator approval - good if you have active mods that take care of filtering your forum's content
- FoF StopForumSpam - checks sign up data against StopForumSpam's database to check for reported spam
- FoF Spamblock - mark user as spammer, suspending them & hiding their posts and discussions
- FoF Disposable E-mails - prevents users from signing up with disposable emails
- FoF Ban IPs - ban IP address of an user
- Automoderator - for example, assign a "Trusted" group to active members that bypasses the need of their posts being approved
All of these extensions are available at FreeFlarum. You can also search for existing discussions about spam here.