If a creature without trample over planeswalkers is attacking a planeswalker, none of its combat damage can be assigned to the defending player, even if that planeswalker has been removed from combat or the damage the attacking creature could assign is greater than the planeswalker’s loyalty.
June 17, 2011 · Commander 2011
rule 702.19f not present in this release
April 1, 2011 · Duel Decks: Knights vs. Dragons
An attacking creature that's removed from combat is also removed from the band it was in.
October 1, 2010 · Scars of Mirrodin
An attacking creature that’s removed from combat is also removed from the band it was in.
February 1, 2010 · Worldwake
An attacking creature that's removed from combat is also removed from the band it was in.