For example, suppose Foo.app is a fat application bundle installed on a PowerPC Mac. While logged in as an admin user, you would open a Terminal window and type something like:
ditto --rsrc --arch ppc /Applications/Foo.app /Application/Foo-ppc.app
Now you have a new application called Foo-ppc.app. Test it. If it works, you can delete the original and rename the stripped version. To strip PPC code from binaries on an Intel Mac, you would change --arch ppc to --arch i386 in the above command.
In case it's not obvious, keep in mind that this operation is not reversible. Once you strip the code for a particular architecture from a multi-architecture executable, it will no longer run on that architecture. So keep a backup copy.

