Used to be a lot of people understood and knew how to take no for an answer to this kind of situation. These days you have to say it repeatedly, then draw them a picture, and possibly sign an offical waiver to close the deal.
Before I left my job the departmental manager would lean pretty heavily on people who would not volunteer to donate each time the "blood bank" came around. They were a lot looser on flu shots, but real sticklers on giving blood. In a way, work beign a good place to give blood has some poetic justice. Visions of "who!, who does not want to wear de yellow ribbon?!" come to mind. The last opportunity, I literally got called into his office to be quizzed on why I wasn't donating. I firmly stood my ground. When that didn't work, I lied; that worked.