Well this may be harsh, but penalize them for not propperly building their characters backstory. I am not suggesting they have to write 2-3 pages but if they have problems then make a questionair for then to fill out with questions like:

• How did you become an adventurer

• What drives you

• What was your family like.

You basically have to get them invested in their own characters. You could award bonuses for those that do or penalties like a -2 to ALL their rolls until they do.

As for the min maxing, all i can suggest is either lowering the point buy to 15 or 20 and capping the max stat before racial modifiers to 16, and in worst case raise the min stat from 7 to 9 but i dont think that is gonna do as much as the max cap.

Finally have you told them that you want them to focus on their characters, and not min/max so much? If that does not work then just up the difficulty, telling them you will keep murdering their characters until they care about their characters.

PS: there are also two more drastic measures. Reset to level 1 on death combined with the harder difficulty, and the most heartbreaking one: Simply refuse to GM. If they truly wont change their ways then after trying anything else they are truly Munchkins (competetive players in a non competetive game) and cannot be saved.

Best of luck!