You can cut it down to size, but I wouldn't use a hacksaw unless you have a steerer cutting guide. Get the local bike shop to do it. The amount that needs cutting off = the total height of your locknut, and then minus 2mm so that the locknut lip is not tightened right down on the end of the steerer.
And also make sure you measure twice before getting it cut.
You can cut it down to size, but I wouldn't use a hacksaw unless you have a steerer cutting guide. Get the local bike shop to do it. The amount that needs cutting off = the total height of your locknut, and then minus 2mm so that the locknut lip is not tightened right down on the end of the steerer.
And also make sure you measure twice before getting it cut.