Help request: How to block txt msgs from unknown senders on Galaxy S9?

ruouttaurmind thought this was worth mentioning said

I’m trying to help an associate who has become the target of a massive number of text messages from unknown senders (several dozen a day). This activity is overwhelmingly limited to txt msgs, very few phone calls.

Blocking individual numbers has proven futile since the txt msgs are coming from many different phone numbers (many hundreds maybe?). They are unwilling to simply change their phone number.

I’m an iPhone user, not familiar with the Galaxy S9 interface and capabilities and therefore at a disadvantage.

Is it possible to block all incoming txt from unknown sources? Accept text messages only from senders on a whitelist, or already listed in the contact list?