So I needed help with an intelligence rune. So I go to my local hireling vendor and buy a level 1 wizard. Her listed spells were acid spray, shield, and I think magic missle. All I wanted the hireling to do was to open the rune door for me. So after she did that I clicked her inactive button. As fate would have it, the battle was a little close to the hireling and even though she was inactive, she decided I needed her help. So ... she casts grease which causes me to fall. Then she casts burning hands, which causes me to catch fire.
Fortunately, I escaped with a few paltry hit points and the monster I was fighting was destroyed. But, do all hirelings have spells other than those listed? I specifically wanted to avoid a wizard casting grease on me. It did look kinda cool though!