seems so simple, but...

 From:  Frenchy Pilou (PILOU)
5400.15 In reply to 5400.14 
Sure :) But it's only one method, surely so many other!

The key for trim anything is that ;)

A curve (surface, volume) for "trim" a volume!
With Trim = the volume stay "Open"
With Boolean Diff the volume stay "Closed"

Above i have only used a curve :)

PS A circle is sufficient even the Diff Booelan here trim the 2 sides of the piece
Just kill after the function the hole that you don't want (it's 3 clicks if it's an inner wall volume, one click if it's a surface ;)

For avoid that you can take a cylinder :)

EDITED: 17 Sep 2012 by PILOU