1) LJ automatically tags each of these autoposts "twitter"
2) You can exclude posts from your friends list based on keyword.
For instance I just went to:
told it to "Exclude selected tags" and selected "twitter".
Of course, if they post a thought about Twitter then I won't see that if they also tag it "twitter". But hey, you win some you lose some.
Original post on Dreamwidth - there are comments there.