There is legal requirements if you are on staff of a theater about recommendations.
Talk to you HR person.
But basically, if I do not recommend someone, I would never say "OH MY GOD, DON'T HIRE THIS PERSON."
My recommendation would be all factual.
This person worked on this show, these dates. They were terminated at the end of the show, and not re-hired since.
Someone would get the idea.
If the person was an INTERN under me, and someone asks for a recommendation, I will ask them to explain the job to me, and then I will say where they had successes with me and my team in areas that line up.
Remember, that if you saying something that could harm them, then you might get in trouble.
I also recommend giving recommendations either in person or on phone and not emailing.