You can mute another player if you no longer want to receive messages from them or see their posts in Global chat. To do that, you can tap on their flag in any of their messages to open their profile page and select "Mute" at the bottom of the page. 

This stops you from seeing their messages, but does not inform them they are muted, they will just keep messaging as normal. If you change your mind you can see the list of players you've muted and unmute them in the "Muted List" which will appear in chat when you have someone muted.

If they execute one of your nobles, send you a tribute or a proposal it unmutes the 1:1 chat so you can see that action and respond to it.