To Block a number in Android from the contact window, click the three button menu at the top right corner of the screen and select "Block number." Callers will hear a half ring and then go straight to voicemail. You can decide if you want to listen to the VM or not.
You can get to the Contact window by clicking the person's name or number directly in your call log or by clicking the three button menu at the top corner of their message screen, go to People and Options then click their name again.
Text Block for Flip Phones:
In the "To" field text 9999. In the message body type "block" then a space, then the number. This will block the number from texting you, even from a flip phone.
We've been talking about Android phones but for iPhone users who may be reading, be aware that selective text blocks will not block iMessages. You'll need to do that from iTunes (instructions for that here).
From your My Boost account:
Log into My Account. Select the “Settings” tab. Select “Block Text Messages” under the “Service Settings.” You can either block unwanted messages from individuals entering in a single number to block or you can block all incoming text messages by selecting “Disable All Incoming Messages.”
Boost also has a paid service called Call Watch that can help block robocalls and other hard to stop calls.