On Windows 7, it seems that the "you have died and will respawn in x seconds" notice takes up foreground control and disallows using the console, or even quitting the game in the normal manner by hitting ESC and using the in-game menu.
The server I joined may be intentionally setup so that people spawn and die in the middle of the aliens, cyclically, and essentially get trapped in-game and on the server unless they use OS-level means to close factorio.exe. Clever. The respawn notice shouldn't disallow the normal means of quitting the game or disconnecting from the server.
If you'd like to see what I mean, join the server in my screenshot and give it a whirl. Maybe this isn't an issue on mac/linux.
http://imgur.com/a/q6pQ4
EDIT:
I found this:
viewtopic.php?f=30&t=41187
But it only mentions console use, so I want to be sure that the game menu is taken care of as well, since I couldn't quit the server. I don't know console commands, and my "maybe it's like id software and /quit will get me out" thought is the only reason I know the console didn't work.
[0.14.22] Can't quit/use Factorio during server respawn
Re: [0.14.22] Can't quit/use Factorio during server respawn
Fixed for 0.15.
If you want to get ahold of me I'm almost always on Discord.