There's a few ways to get round it on iOS 6. One method would be to backup your device through iTunes, look at the backup files, open the iMessage database, remove the message and restore the device.
You can perhaps get the offending sender to send more messages or even just a long message and that will move the text off screen and allow you to delete the conversation.
You can also sign out of iMessage, sign in as someone else and sign in again to wipe all past conversations but you may not want to wipe old conversations.
If you want to keep old conversations, it may be possible to setup iMessages on OS X, sync the phone with it, remove the message, wipe the message log on the device by signing out and then syncing them back again.
It's been fixed in iOS 7 and 10.9:
Hopefully Apple will issue updates for every OS affected and in future, they should run text processing in a separate process if possible so that applications don't crash and offending strings would just cause Core Text to go down.