'Painter' stained our expensive beams incorrectly. What are the options for fixing?
Have you been in contact with the company that did the original job? I'd certainly say it's their responsibility to fix the problem at no additional cost to you. 

As far as the method of fixing the problem, sanding is probably your best option. When I was figuring out how to properly stain test scraps, I made a few mistakes (see this question). Just to check if I could correct mistakes on my actual project, I sanded down the test scraps and re-stained them. Afterwards, I couldn't tell they had been refinished. (Note: I was working with pine - not sure how cedar will react.)

Just make sure whatever sander you use has built in dust collection, or use it in conjunction with a shopvac. You're going to have a lot of dust from the size of those beams.