I go online to My Verizon account and do it that way. If in doubt or don't have access to the internet, I would think a Tech Support Rep (611) could help with that issue or when your out and about stop by a Verizon store and ask them to help. I do know that you are able to block calls and/or texts from certain numbers. Hope this helps you.
If you just want to block text messages you will have to use a 3rd party app (Many on Market) but if you want to block all communications from the user you can do this at https://nbillpay.verizonwireless.com/vzw/secure/messageIntercept/messageInterceptHome.action , this area allows you to block all communications from any unwanted users..
Also consider trying a 3rd party SMS like handcent that has a option to block these text messages..