are you saying the rust keeps coming back? or you can't get rid of the rust in the first place?
The answer to the first question, is probably the second. If you don't get rid of all the rust, it will grow.
Completely disassemble the bolt and really clean it. you may need to go to a chemical cleaner like naval jelly, but I don't like using acids on my guns.
i prefer RePete's method. Disassemble, Clean really well, and oil.