Holer Moler
The Answer is to legislate in the same way as Aircraft that require Pilots.
1. The Drone/UAV must hold a certificate of Airworthiness.
2. The operator/Pilot must complete a recognised course and hold the appropriate licence.
3. And if the Drone/UAV is being used in a commercial sense - an Air Operators Certificate must be obtained. And the Pilots must obtain a commercial licence.
4. Lastly it should be enforced that the companies or manufacturers of these products are not permitted to sell these Airborne devices - unless the purchaser can prove that he/she complies with the above.
Thoughts !!!!!!
Let's go to the other extreme.
Just like radio frequency spectrums that must be shared (after all the air is free isn't it)
Drone operations need be regulated to operate only in certain airspaces. Even those without training or credentials (the guy next door) can operate in some spaces without having to show a paper trail. And the big guys can operate in the same space as well but at their risk.
So that leads us back to the fundamental question of what risks are quantifiable (not this what-if stuff, that fills the evening news and discussion boards).
Somewhere along the line we are going to have to assign responsibilty between both conflicting users of the airspace to avoid collisons and for that we need reasonable wordings not just "keep off the grass" signs.