- Remove them completely.
- When you gain an Ability Score Improvement, you can only raise one attribute one point.
- When you gain an Ability Score Improvement, you gain two points to spend. It costs a number of points equal to the ability modifier you'll go to if you raise the attribute one point. (For example, if you were to raise an ability score from 14 to 15, it would cost 2 points since the modifier for 15 is 2. If you wanted to raise the attribute to 16, it would cost you 3 points since the modifier for 16 is 3.) The cost for negative modifiers are as if they were positive, so raising an attribute to a -1 modifier would cost 1 point, not -1.
Personally, I'm leaning towards option 1 or 2. Option 3 sounds interesting, and would keep the higher ability scores harder to get to, but it may be too much book keeping.
What do you think?