How to prevent students from using modified calculators to cheat on exams?

Just don't allow calculators.

Or design your exams such as you can't cheat with a calculator

You'll just have more work to do to create exam that test the logic and reflexion without big calculations.

I think it would lead to even more interesting subjects.

In France, we do two years of math / physics preparation before entering in engineering school, and we're never allowed calculators in maths. And french engineers are considered to be very good at maths

    Additionally, students using them often don't need to learn how
    to perform various calculations and can instead just plug in the
    various values and have the answer come out.

If they get full credit for doing that, it's a test-scoring problem not a calculator one.

Sliderules alround