Man, spam is just the worst and in your situation where you are having to limit the amount of messages you send/receive, it makes it even more frustrating.
There may be hope though as most short code messages use the same sort of START/STOP functionality if they are being sent to you automatically. You may be able to reply to one of the messages with STOP and that should trigger the automated system to discontinue sending you messages. If successful, you may get another SMS from that same short code saying you were removed from a list or something like that. If that does not work, you may want to see if you can find out what these messages are about and see if there is any contact information with them where you could then call the company and asked to be removed.
Ultimately, changing your phone number would stop these unwanted messages but that is always a last resort.
Thanks, Chris. I had heard about replying to such text messages with Stop but I had also heard that with email spam, it's not advisable to click the included unsubscribe link if the email looks sketchy and I thought the same was true for SMS spam. But after you advised me to try it, I did, and it worked! In fact, I only replied to the short code that was sending four copies of the same text (to hedge my bet, so to speak), but the texts stopped coming from both short codes! Thanks again.