Block texts not in contacts

Firstly you could do this to avoid seeing such messages:

In the iMessage app on the top left tap on "Filters"

In there tap on "Known Senders". This will only show messages from people in your contacts as long as you keep to this filter setting.

Later on, once a week perhaps, you could tap into "Unknown Senders" to review and delete all those texts not needed.

Another way is to download a call and text blocker. They rely on databases of known scam, nuisance and junk callers and texters. This service usually involves a subscription, and they may or may not be effective enough.

Hope that helps.