number one is not always possible. Many SMS apps only allow you to block phone numbers, not email addresses, which are where most of these spam texts come from. Reporting messages via short code also won't work as the response will be a carrier request for the phone number, and since again, these are emails being sent to phone numbers via an email-to-text gateway, the email address in response just reproduces the same request for a phone number over and over again.
Best means of getting rid of these are 1), don't give your phone number on any online form, as many companies sell it to god knows where, always provide them a fake phone number. They don't have a legitimate reason for your personal phone number anyway. Many companies only correspond via email for things such as account and billing notifications, so the phone number is just sold to the largest bidder.
2), don't respond to them. Ultimately they're fishing for a response to confirm it's a live number, often just dialing out random numbers hoping for a response. Eventually, they cease.