I have run into this problem quite often since it's so hard to buy bouzouki strings in a small town. I buy individual strings of the proper guage which are only available in ball ends.
I take a pair of vice-grips and set them a little smaller than the diameter of the ball, then squeeze the ball, making sure not to squeeze the string loop. I usually have to squeeze it into an oval, then squeeze the oval from the long ends. This will cause the ball to crumble into little pieces and leave you with a loop end string. I've never had any trouble doing this and have never broken the loop on a string.