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
Not included in the above, but handy is also an alternative web UI for Reuters news: https://neuters.de