You can't cancel messages already sent. If anything it should be done at your contacts computer.
I think it will also be a good idea to enable CEIP, to help Microsoft solve these kinda problems. Read
Why Microsoft Needs You and Why You Should Care.
You can also try to find help on
The Windows Live Messenger Newsgroup. If nothing there you can post it..if anything it will bring this problem to their attention.
Edit: i already put a similar/connected issue (large amounts of text/emoticons freeze Messenger) on Connect a little while back but it won't hurt to bring it up on the public newsgroup aswell.
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