Spam messages in Signal

enable "block unknown" just below the "blocked users" in settings

I've never once had this issue while I get normal SMS spam. My grandpa and wife get them though. Still trying to figure out why. I'm about to make the switch to Molly as I read you can allow messages only from an approved list of senders or something along those lines. Molly is a fork of Signal FYI, and you'll need their fdroid repo to download or get it from github

I get one about once-a-week, I just block and report as spam. There's no automatic option that I've seen. I wish there was; my parents use Signal and I worry about them getting scammed. Thankfully they are good with asking me if they get something that's out of the ordinary.

You can report the message so that future messages from the spammer won’t send. Unfortunately no direct way to mark the message as junk automatically like email, but Signal does have Message Requests which may help? https://support.signal.org/hc/en-us/articles/360007459591-Signal-Profiles-and-Message-Requests

Did they get your number to send spam?

One of the secret sender options is to disallow messaging from anyone (I think it's on mobile), though I am not sure what portion of spam this blocks

I'm not aware of any functionality which allows to block messages from unknown users. I think blocking them might be your only option right now.

Easier said than done (and already too late for you), I know, but: in general, be careful where you leave your number...