Select the spikes in your image editing program, invert the selection, then save the selected area as an alpha channel. Save the texture in TGA (Targa) format. I think TIF will work as well, but don't quote me on it.
Import the texture, the spikes will be gone.
You may want to leave those spikes there, though, Angerfist. Because if you remove them, your new character will have big holes in his skull:
Caveat: when you make part of the mesh invisible, that part shows up when the character is 'highlighted'. For example, I once made Frostbite's 'armor hood' invisible, but when I picked up the double damage powerup his body flashed with the purple energy, and you could see the energy crackling around the invisible part of the armor, though the armor itself remained invisible.