It’s possible to keep a spider out of a directory that’s called “customers” or a directory like “images”. Obviously it’s a request, not a guarantee. And if you don’t want the file /secret1960 to be found by anyone, maybe it’s better not to put it on the internet anyway, and especially not in your robots.txt file. If you want to protect your information you use a password security like .htaccess or a login-script.