Despite several answers to this same question, I have been unable to successfully remove a material from the materials list in my projects. Following answers in other questions, I have gone to the Properties section and opened the materials tab, then selected the material I want to remove, then clicked the X button next to the list to unlink. I've also tried this with a shift click to set the users to zero. After a save and reload, the material is still in the list.
Since I believe I've followed the instructions in multiple answers to the letter, I am forced to conclude that there is something different about my project. The material is a direct linked library datablock. Perhaps this is the cause? If so, I'm guessing I need to remove the link between the external file and this project.
Another answer suggested to click the make local button next to the list, but this seems to create a local copy of the material, while the original remains linked. The local copy can be removed following the standard methods, however.