The pipe cutter idea works ace on bars, steerers and seat posts, not sure you'd have the same success with a frame though. Might be ok if you're cutting below the clamp slit, but that'll cause you all sorts of issues if it needs to cut over it.
If you use a hacksaw, make sure you have a fine tooth count, it'll take longer but you'll cut smoother. And also be careful not to cut into your hose clamp guide, if anything cut slightly above it and file it down.
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more good advice.....you guys will make a mechanic out of me yet