If we punish pet owners who have not spayed or neutered their pet when thy get a fine they will likely give up the pet, or if they don't pay the fine the pet will probably be taken away from them and put in a shelter, defeating the whole purpose of the plan.

Instead we could register your pet if it is spayed or neutered and be entered to win a prize each month, say $1000 or take their pets picture for a calendar and sell the calenders. Point is there many rewards people would go for that would save money and most importantly, pets!