Originally Posted by DaFace
Depending on how old your phone is, you can send calls straight to Voicemail without any extras. Make sure the number is added as a contact, then go to edit the contact, pull up the menu, and select "all calls to voicemail."
Not sure about text blocking though. Here's an app that does both (and more) that appears to be well-reviewed, but I haven't used it personally.
on mine, you can longpress any entry in the call log and add it to a reject list, doesn't matter if its added as a contact
you can even reject all calls containing or starting with any numbers you choose