There seems like there have been spam waves on Mastodon lately. Trolls that just got nothing to do but stir up troubles with allegedly innocent Mastodon instances that are hosted by independent hosts.
What do spammers do? Unlike trolls who come in to have real conversations but in a way to stir up drama, spammers will join a Mastodon server and just spam post the servers.
What Mastodon instance owners can try to defend from spam-
Servers with instant signup policies face the greatest server overload risks particularly from spammers, if they don't put in security measures such as setting manual review sign up, banning throw away email addresses (scammer tend to use), or setting bot checks.
Lesser known servers luckily remain the least affected by bots. If your server isn't listed on the official Mastodon instance page for finding an instance to join you may be less likely of encountering spammers in your server. This is because if it's on an alternative Mastodon instance search tool but not on the official, most people won't see your Mastodon instance as easily and be able to join it if they never find it.
Spammers must be stopped. Surely you wouldn't spam infesting your server as a host or even- a user.
What users can do.
If you notice a spammer on the local server you are on, report the spamming account to the servers owners and moderators if they don't know about it yet. After that, the user can just block that specific spammer and you won't see from them again. Atleast not that account. If spammers are rampant on your instance the owner might have to make drastic changes to how it handles new account signups.
Rob200- I hope this has given insight, and clarification to the whole situation revolving around spammers, and if it has impacted you, open signups might be the main culprit or if it's on popular search tools like the default Mastodon sign up process.