Write a birth plan, discuss it with your doctor, get it signed, and have copies. If possible, get a copy placed at the hospital beforehand and also in your doctor's chart for you. *And* have copies with you when you go to the hospital.
You absolutely can refuse anything you're not comfortable with, but, if I were you, I'd pick your battles. Decide which things you're willing to put up with and which you aren't. I'm not saying to give in on things that are important to you, but compromise will probably help your doctor and the hospital staff to be happier with you. Many would say that it's not your job to make them happy, and it isn't, but there is a reality that you want a good relationship with your medical staff.
Expect nevertheless to have to say no alot, repeatedly and firmly. And politely, for that matter. But don't let them push you around on the day. Any compromising, do *beforehand*, on your birth plan. If the nurses insist on arguing with something, refer them politely to your signed birth plan which your doctor endorses. And as much as is possible decide ahead of time what you'll do if circumstances make it necessary for you to change your mind.
I'm in a pretty similar situation to you, actually. And the hospital I'm giving birth in is like a relic from 10-15 years ago, as far as certain things go. So, I offer these things on the basis of my experience with my mother's last two natural hospital births (13 and 15 yrs ago), and on my preparation for this upcoming birth (I'm 36 wks now). Good luck!
ETA: About IV's- this is one of the things my hospital gets real shirty about, but I won't let them hook me up to one- partly because I don't want to be tied to a stupid stand, but also so they can't just slip some drug in there. They wouldn't let me refuse one without getting nasty, but we compromised on a saline lock (basically an unconnected IV port in your arm). Might work for you too, if they get intrasigent.