[khanar]

It's usually religious

It's usually religious beliefs.
For example Jehovah Witnesses who are not allowed to get blood transfusion due to religious ban.

Or that classical scenario when you have a patient who's thinking that he/she got this disease as punishment from God.

Or that patient who wants to pray so that the disease will disappear ...etc

 

[khanar]

Jehova Witness is a special case

ok.. So if some guy says that he would not get a transfusion, and he really needs it, do you try to convince him that its important, or do you not pursue it at all if he quotes religious reasons?

Blood transfusion is an exception because it's a life saving measure at most of the time and you have to act in the patient's best interest. You should tell him the consequences of "not having a blood transfusion" and if he/she insists then you don't give the transfusion no matter what.
The only exception is that if that person was the guardian of a child and he/she refused you giving the blood transfusion to his/her child then in that case you give saline and try to get the ethical committee or a court order as soon as you can.