Well on my Paladins I do not have Shift+1 bound to anything so that key does not get broadcast. My Warlocks however do have Shift+1 bound to something. I tested this on my Paladins on my login screens. Upon toggling the key maps off [pressing 'pause' once] it is working just fine...nothing is being broadcast. After toggling the key maps back on [pressing 'pause' once more] now Shift+1 is being broadcast to all windows when there is no such key map on my Paladins.

EDIT:
Tested in game as well. After my Paladins have loaded up into the game I turn on keyboard repeating to open up the chat box on each of them, then turn off keyboard repeating. With all their chat boxes open Shift+1 is of course putting !!!!!!!!! in each window's chat box when it shouldn't be.