Don't know what PIN they could mean, but unless it's a PIN associated with some account you have WITH THEM, no way I'd give it to them.
You are able to selectively block phone numbers from sending SMS to you via the Sprint Web site. Are all the texts from the same number? If so, just blackhole it and you'll never see them again.
Log onto the sprint website. Click on the "My Preferences" tab, and under "Limits and Protections", click on "Block Texts".
