I deleted some scenes in my .blend file, but now it is larger than it should be because it has a lot of unused materials in it. How do I get rid of like 400 unused materials?
Assuming the material is unused (i.e. it has the circle next to it), then you should save the file and then exit blender and reopen the file or just use the reopen last command.
Yes, I reopen the file- the unused materials ARE gone i.e. they are not in my material list, but in my OOPS Schematic I see hundreds of blocks e.g. tex.186 or whatever and there are hundreds of them.
How do I get rid of that?
Please people, dont everyone reply to my post at once. Take your time.
K, no one’s figured this one out yet. What I did was start a new blend and append the Scene from the old one. See if that works, or appending all the objects seperately.
Ohh sarcasm. Ha! You think you’re entitled to an answer?! Figure it out yourself, and then tell the community. The program works, and the filesize is normal (it’s not like they are still taking up bytes but you can’t assign them, they are actually gone.)
So this is really a cosmetic issue about your Outliner? Again, why don’t you figure it out for yourself (since it bothers you) and then tell us. 
Welcome to OSSI… this is where the word “initiative” comes in.
While you’re at it, could you compile every option for Yafray shaders and what it does into a concise, easy-to-read document? And translate it into at least 5 major languages. We could really use that too. 