Using loctite is a bad idea, and totally not necessary. The problems with it are 1) It makes it hard to change props and 2) props can compress over time (especially wood ones) and you won't be able to put a wrench on it and tell because the nut is glued on. Your problem is simply that you aren't tightening the nut down enough.
Instead, use a box end wrench or at least a big pair of pliers to torque the nut down hard. Don't worry- the crank won't break. Don't use a spinner with a plastic back. If you must, put a piece of open mesh plumber's sandcloth between the spinner and your prop so you don't have plastic rubbing on plastic. Better is a metal backplate that really grabs the prop. Tighten it down hard with the wrench and it won't move until you want it to. If you ever use a wood prop, retorque the prop nut periodically to keep the clamping pressure up as the wood compresses.
BTW, the only reason a 4 stroke needs that second nut is in case of backfires. A backfire puts incredible torque on the prop in the unscrew direction, so much so that they have been known to quickly unscrew all the way off and go flying through the air.