i will say though that if the user sending them is on VZW...they may not get a message back saying you blocked their text...but im going to assume they will be able to tell that the message was not "delivered" to you...
From what I uderstand, cellphone numbers are private and are not listed in any directory. So knowing that, you will have to assume VZW and other carriers are selling your number to advertisers for profit.
And IF they are selling their customers numbers then I would imagine it is a clear case of invation of privacy and with enough complaints, and I meen TONS of complaints, they will have to stop this practice AND give you a new number on their dime.
Of course we are talking about a multi billion dollar company here so good luck with all that....lol