you have to apply pressure to it so that it is forced into the center link more.

basically it is tapered. so it has pushed back far enough that the centerlink isn't providing any friction to stop it from turning.
press the two parts together by hand or say a strap to hold enough tension on them so it stops it from spinning until it pulls in far enough on it's own.

if it still doesn't stop spinning something is wrong, perhaps the taper got worn down or the hole got wallered out because it was loose. but generally that is all it takes.
no castle nut and cotter pin?