I don't have prepay but I think http://www.verizonwireless.com (My Verizon) has a section for pre pay customers.
You can then register for My Verizon and one of its features is to block 5 numbers/text msg from bothering you for up to 90 days. you then go back and re-enter the numbers again after 90 days. They have a service for $4.99 a month called Usage Controls which can block 20 numbers permanently as well as unknown numbers and international etc. I forget the rest but the information is on the Verizon web site.
The option to block numbers via the on-line MyVerizon isn't available for prepay accounts. Prepay customers can have blocks put on their phones by calling Customer Service:
Customer Service - Prepaid
Dial *611 from your cell phone
24 hours a day, 7 days a week
You really come alive in the Spring.....
I called and I just spole with Customer Service. You cannot block individuals numbers on prepaid they told me it takes too much cost on their end.....Which is **. They said only option is to change yout MTN (mobile telephone number).
Or you can get a post-paid account which has that feature. It's your choice.