Boost does not have an account level selective block for voice. Instead, you can do it from your native Android app instead. Blocking from Android applies to calls and texts. Calls ring once and go to Voicemail, texts are sent to a "blocked senders" folder. As you can see, it's more of a re-routing than it is actually blocking, but no notification is sent to your ex BFF, they're just shuffled off to a place you don't have to see if you don't want.
To Block a number in Android from their 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. No direct "this person is not speaking to you!" but they'll likely catch on after a while. gain, no message is sent alerting the other person that you've got the blocked. 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.
If you block someone from texting you from your Boost account settings, they get a message that you have chosen not to recieve messages. Though it doesn't say 'chosen not to receive messages FROM YOU,' your ex BFF will probably know you blocked them. Still worth it if you're being harassed or abused. I know you were asking about this on an Android but for iPhone users who may be reading, it should be noted that selective text blocks will not block iMessages. You'll need to do that from iTunes (instructions for that here).