Why would you be obligated to continue working for this company after you no longer work for them? You have done enough already. Time to talk to the boss, tell her all that has happened, and that you have moved on and you are now busy with school or anything else that you are doing now. If they want to contract with you for services, they can come up with terms that are acceptable to you.

I'm sorry you were treated like that. You could ask a lawyer if you have a case for "hostile work environment," but if they are such a small company, it might not even be worth your time. There is no excuse for how your co-worker treated you, though.