I assume your talking about the collet adapter that goes onto the bare shaft, the one that squeazes down on the shaft when you tighten the prop nut, right? (Since its the EF Yak, its the type you should be using.)

Clamp a 9/16 wrench into a vice with the open end of the wrench close to the vice jaws (less flex).
Insert the "collar" part of the adapter in the end open end of the wrench and let it set against the lip on the collar.

Then use a piece of hard wood to protect the end of the shaft while you gently tap on the threaded end.
It may actually take a good whack or two.

Keep the nut on so it doesnt come totally apart and hit the floor, and devise a way to catch everything if it does come apart, especially something to catch the motor if it comes loose.

All you need to do is to get the collar to loosen, that will release the clamping on the shaft.