simple solution... if they don't want to join the union, they shouldn't be entitled to any gains made by the union. if the union negotiates a pay raise, non-union member should have to negotiate their own raise. if you are part of the union, you get union benefits. if not, negotiate your own benefits. pretty simple solution.
the strength in unions is in numbers, aka collective bargaining. take out the numbers, bust the union. in other words, without the ability to collectively bargain, unions have zero power.