When a player is typing a message, make him "(paused)".
There that should solve chat blocking.

Edit:
Nevermind this idea. It would just make guilds run into flag room or next to the blockers and start chatting till the rest of their guilds gets there.
Unless someone can think of a way to prevent this from happening.
I.e: being able to take damage while being paused, but still won't block the path.