how do they decontaminate radioactive materials?

Just read that Marie Curie’s notes were radioactive, “until they were decontaminated.” Is this just a matter of washing off the radioactive dust, or is there something more elaborate and nuclear involved?

The only decontamination you can do for something like that is washing off radioactive dust. If the material itself has been partly transmuted into something radioactive, you’re not going to be able to reliably un-transmute it. Well, other than by waiting a really long time.