Explain why you cannot just take away a proton to make a halogen negative.

The force that holds protons and neutrons together is too strong to overcome.

Consider the location of the particles in an atom.

Protons carry positive charges and repel each other. The nucleus will break apart without the strong force that holds the protons and neutrons together. This force is much stronger than the attraction between the nucleus and the electrons. X-rays are energetic enough for removing electrons from an atom. However, you'll need a collider to remove protons from a stable nucleus. You could well have ionized the atom with all that energy.

Also, changing the number of protons per nucleus will convert the halogen atom to an atom of a different element. Rather than making the halogen negative, removing a proton will convert the halogen atom to the negative ion of a different element.