After receiving another text from 600-600, I tried replying with "stop", as I saw recommended a couple places. I must say, being primarily used to email - where replying to spam often means you're simply confirming your address is active - I wasn't keen on it!
Anyway, it worked - got a response that they are stopping. Turns out "600-600" is "rivals.com" - is there not any way to cross-reference numbers to businesses? The messages they were sending sure didn't have any identifying features...
You followed the correct steps for getting removed form a list by using one of the standard replies:
The best way to unsubscribe from unwanted recurring Premium SMS Text Messaging is to “Reply” to the message, input STOP, END, QUIT, CANCEL or UNSUBSCRIBE from your phone and Send to the content provider's short code. The content provider is supposed to recognize these standard responses.
As for the codes there is no definitive list, but one of the best ones I've found for US Short Codes is here: