Prevent iPhone from trying to send text message after leaving airplane mode
There's a hack that I've used to stop a message from being sent.  I presume it worked because I didn't get any blow back from the excoritating message I sent to the wrong person (sent to client, not to tech who went the extra mile to find ways to pad billable hours)

Enable Airplane Mode as quickly as you can
Settings →  Messages → Turn off &quot;Send as SMS&quot;
Return to your messages and delete the message (hold the message, select &quot;more&quot;, then select the message and then &quot;Delete All&quot; at the top
Disable Airplane Mode

The key here is to enable Airplane Mode before your message hits the Apple servers.  Once it's there, it's out of your hands.  As for blocking the person, that's for incoming calls/texts, not outgoing.
Now, Apple is supposedly testing new iMessage features including retracting messages, but we'll have to wait and see.