I have been waiting to see this feature added in messages app. In India we get lot of messages from senders (banks, utility providers, service providers) with no phone numbers. Samsung messages app can't block these messages as there is no phone number of sender. All other messages apps in the market are able to do it except Samsung messages. I really don't understand why this is not handled by Samsung.
Use SMS organizer by microsoft , u can mute the sender. I've been using it from 4-5 months and its very good.