Why is it a 405 in the first place? I had to go look it up: Method Not Allowed. But here it sounds more as if you simply don't like the visitor's face. So why not a basic 403?
File does not exist: /home/myusername/public_html/405.shtml
Frankly this worries me more. Why is the server looking for 405.shtml? It's got nothing to do with the 405 as such. The error message implies that somewhere else you've said
ErrorDocument 405 /405.shtml
and then when the server encounters a 405 error and goes looking for the document "405.shtml" it can't find it.
For starters, check your typing. I noticed this post is in the AdSense forum and the subject line says "indix.html". OK, so there's nothing wrong with telling the server that this is how you choose to spell it-- but it's unusual enough that I have to wonder about typos along the way.