Currently, when a player casts a spell that they don't have enough Psi energy for, they get damaged, but there are no other real consequences. This is fairly abusable when combined with Resistance or increased health, or both.

Rather than damage (or perhaps in addition to it), I'd suggest letting the player's Psi energy go negative, disabling their ability to cast until their Psi energy is positive again.

It would probably work better if you simply didnt allow a player to cast if they dont have the energy. (But that will also result in less halarious psi overdraw deaths)