Changing your number is one way, probably the most effective, but may not be a preferred option for you. Some phones have a setting to allow calls/texts from contacts only. This may or may not work for you.
If you have the Eris, there may be apps that allow for different variations of call screening so you don't have to deal with them directly. You can put a block on Premium SMS messages and that may stop some of the spam from getting through.
If these texts are a 5 digit number they are premium text messages. I advise you to call and have the Block Premium Text Messages put on your phone. Have them check your account for any subscriptions that might be on your phone and possibly get a credit for it. Depending on how long it has been going on.