Use the Spam controls - you can block up to 5 numbers for free for 90 days, and if you need to extend it after that time you can.
Quoting from Verizon's FAQ's on Spam Controls:
"Call & Message Blocking allows you to designate and block up to 5 phone numbers from sending text, picture or video messages to you.
You cannot block numbers that are within your account nor can you block non-10 digit numbers such as 911, 411 or #MIN (to check minutes used)....
Blocking will expire 90 days from the date of your first block. You can reestablish blocking by adding blocking preferences after they expire."
I'm not sure if this blocks OUTGOING calls and texts to that number....
What kind of phone do you have? On certain phones you can restrict outgoing and/or incoming calls to only the numbers you've saved in your phone's contact list. Not all phones have this feature, but enough do that it's worth a shot. This feature is usually enabled from your phone menu. Check Settings & Tools > Phone Settings > Security (at this step you'll be prompted to enter the phone lockcode which by default is the last four digits of your mobile number) > Restrictions (again you may be prompted to enter you lockcode). There you should see options for Outgoing and Incoming Calls. Change from Allow All to Contacts Only and save your selection. Some models like the enV Touch even let you restrict messaging.