I haven't tried it...but they do sell abrasive blades for hacksaws that can supposedly cut bottles and stuff...so you might try one of them, as opposed to a normal toothed hacksaw blade.
If the bike isn't locked TO anything, load it in a car, take it to a muffler or welding shop, and they can probably have that sucker off in 30 seconds.
A grinder or cutoff saw would probably work as well.