Retroactively removing file from tree

[Stuart P. Bentley]

How can I remove all traces of a file from previous revisions? I'm the  
only developer on my project (with development on a branch hosted on  
Launchpad), and I had committed the binary versions of a few libraries in  
my tree for a few revisions because they weren't yet included in my  
system, but now that they are I'd like to remove them retroactively from  
my branch (and then from the Launchpad branch).