Hi Tom,
that is a strange problem and one I have not yet come across. Before you shorten the spring first check the running duration of the watch, also there is a rule for how much space there should be in the barrel once the spring is installed, I think it may be half or a third?. If after this you do still need to shorten the spring then you can just cut off what you dont need (the spring wont shatter when you do this 
) but you will need to soften the end to punch or drill the new hole. I have found the best way to do this is to have the part of the spring you want to soften protruding from your bench vice, heat it to a cherry red and draw the heat source away slowly. The vice acts as a heat sink and stops the heat affecting more of the spring than is needed. Make sure you clean up around the hole once it is made as this helps to stop a potential tear.
Good luck,
Paul.
p.s. I am sure you already have but have you checked the spring thickness against the original spring?
