the two tips for killing them are
1) shoot them in the face from above and then you can use the spear and their mouth as a handle...this lets you clip all the spines pretty easily
2) use a para tip and have a slider that can push the fish off your spear into the bucket, this way you can avoid touching them at all until you are safe on land where it is a lot easier.
scuba guys have it pretty easy because it is easy to do underwater.
another thing I have noticed is that when I spear them they get destroyed, their bodies bend and they are harder to despine...a head shot makes them flare their spikes out which makes it a lot easier to clip...
there are many videos on youtube of the actual clipping, many neglect the anal spines so heads up