Just like in real life, maintaining our right to say no is very important. You should absolutely never do anything that makes you uncomfortable. Clients are paying for whatever services you want to offer; this doesn't mean they own you.
Sometimes they like to think they are in control, as they are paying for a service, after all. I sometimes wonder if I should report someone when they are asking for some crazy things. You have to wonder if they are serial killers or not.