cut it flush and with the frame then drill the rest out with the appropriate sized drill bit. any other way and you'll likely damage the frame to where it is unuseable.

i've also heard of some ppl using larger cf tube to splice on a new boom, which would be easier to do but takes away from the bling and adds a little weight. the advantage of splicing is you can do it in a way that makes future boom replacements easier (basically you use a plastic tube and a pair of setscrews to make an easily removeable splice).

good luck,
kev