Is there a way to "unsubscribe" from group texts in iOS?
As Messages and SMS integration + iMessage is designed today, this can not be done. 

What you could do is:


block the people and leave SMS / iMessage enabled
place that conversation on Do Not Disturb so you don't get notifications from that group
enable no notifications for either SMS or iMessage
turn off iCloud Messages entirely
disable SMS entirely


Just as you can't block a number or email from sending you an iMessage - there is no technical way to get others to not send you a group message (other than by exerting social pressure to exclude you from further communications). 

I haven’t been able to find anything playing on my phone, and several forum posts suggest that it can’t be done:

On Mac Rumors, sla252 writes (November, 2011):


  Just got off the phone with Apple tech support. They admitted there is no way to remove yourself from an iMessage group chat. The tech guy says the best thing we can do is go to http://www.apple.com/feedback


There’s also a Reddit thread in which huntard713 suggests (October, 2011):


  Settings-Messages. Turn group messaging off.


It's now November 2011 and I can’t find this setting on my iPhone, and this was in response to a slightly different problem (about who sees the other participants in a group iMessage, so I don’t know how effective this.

I’m sorry this doesn’t really answer your question; for now at least, it looks like you can’t leave a group iMessage.