I haven't faced this problem as deeply as you express. Of course, I haven't worked at home since 2007, so that may help a bit.
However, I did have one acquaitence, who thought this was no big deal . . . that the book just "told" you everything you needed to know. With that being the case, and to really sort of show him how complicated coding is, I handed him an ICD-9 manual and some "sample" questions from a certification prep book and said, "go to town." Somewhere around 3 minutes later he gave up, handed me back the book and I never heard another word.
One thing you might consider is when they ask if you can teach folks or want to continue to bug you, give them an "assessment test". Chances are it doesn't even have to be valid, just hand it to them, tell them to complete it and you'll consider mentoring if they score high enough. Now, they aren't going to have the manuals, much less understand most of what's on the test, so the likelihood is low that someone will meet a reasonable expectation. And, when they ask to borrow your books, tell them you have none to loan. Wish them well and then point them toward a coding education source and you've really done your duty. Those who are truly interested know what routes to take and those who just think it's no big deal can find themselves corrected. I'd make a bet you won't get many of those "tests" back. To keep you from having to score them, hand ones out that you already have answers for . . . like a pre-employment test you've taken or something along those lines.
I must admit that nothing quite upsets me as much as peopling treating what I do as if it were answering the phone or something altogether unskilled. I have this friend with whom I've gone to a few social outings. When people ask what I do and I tell them, he "corrects" me and says something like, "he's just a medical coder." It's demeaning (in my opinion) and it's hard for me not to take offense. Oddly, it's always the people who don't do this stuff that think it's easy . . .
Good luck to you all!
Kevin B. Shields, RHIT, CPCO, CCS, CPC, COC, CCS-P, CPC-P, CPC-I