If there is a strong genetic predisposition toward a particular disease, it probably is true that it can not be completely prevented. However, its date of symptom onset, its severity of symptoms, its duration, and how one copes with it can usually be managed by living a more healthy lifestyle.
Yes, I believe there are specific genetic determinants of health that make it not possible for some people to prevent certain health problems. While I can not cite the specific genes, most people are aware that certain health issues are a result of genetic predetermination, and thus could not have been prevented.
I do believe that some genetic determinants of health make it impossible for some people to prevent certain ailments. This is the precise reason why they are putting so much time and effort into genetic studies. We hope to one day prevent the problems created in the genes, so even these people can benefit from treatment.
I used to think that most things, good and bad, were genetic. But this isn't the case. It has now been shown that environment, diet, and exercise can influence the behavior of a person's genes. Even if one has a predisposition to illness, they can overcome this with their lifestyle.