a pair of pliers or a small adjustable wrench is the best/fastest/lightest way, even with a lathe it's a PIA and is VERY noisy to do.

with the plier method, you pretty much can't break the fins off down far enough to hurt the integrity unless you force things too hard, when you grind the remainder smooth don't go too deep, I mainly just smooth them off. Before you get too crazy making things pretty, stick it on a prop balancer, I use the Dubro unit with the narrow part of the taper toward the fixed cone then an aluminum washer on the other side of the flywheel with the other cone on backwards, trim the thing and check the balance, if you get it trimmed and still need to remove material, use a drill and remove a little at a time without going too extreme. They're usually somewhat off balance stock, so don't get too carried away.