On older Android a press and hold on an SMS conversation brought up the context menu options, of which there was only one: SPAM - block this number.

So if you accidentally hold the press for a fraction too long you block that number.

And then you wonder why your friend(s) don't love you anymore...

To unblock it, you have to go to the Contacts and dig through options there, the SMS app showed nothing of help. It wasn't obvious that you had it blocked!