The prop is splined to the prop shaft. If it (blades and all) is spinning on the shaft, the interface between the brass splined core (that contacts the shaft) and the propeller casting (which includes the blades) has been damaged. On a replaceable prop-hub the core is plastic and brass and apparently has sheared. On non replaceable hubs the interface is a pressure fitted rubber insert that is designed to slip when hitting an obstruction and reseat once the operator cuts the throttle to idle and then powers back up resuming whatever condition existed prior. The rubber inserts usually reseat and essentially "heal". The plastic shears and you are dead.
iboats sells plastic/brass inserts to fit your prop-engine shaft. Prop shops can replace the rubber boot. If you are talking about an alum prop on a 100 hp engine (for a number) they sell them on here for about a hundred bucks brand new.....wink! You get all new parts.
When installing whatever you decide upon, nut torque is on the order of 55# at the 100 hp level.