Can you compress the spring if it is not on the car? If you have not tried, remove from car and try.
If you can compress off car (which is likely since you'll have better leverage) you might be able to put a piece of wire around it to hold it shorter. Install, then cut wire.
That spring also might have a self locking mechanism (I don't know). If it does, you might need to first extend it a bit to release lock, then try compressing it.