How do i easily secure subfolders with htaccess

I have a hosting package setup on hostgator
.mywebsite.com

and i have addon domains on the host too
.mywebsite.com/differentwebsite.com

How to i allow visitors to still access differentwebsite.com, but block anyone & also robots from getting to the site via .mywebsite.com/differentwebsite.com

is there anything else i should be aware off?

Ok i've figured it out that i can check the url with my index.php in the subdomain.com and redirect the url to that,

and also in the mywebsite.com i can put a line of code in the robots.txt that will block spiders from going into the subdomain, but what i'm not sure about is how this will then affect subdomain.com? will spiders be blocked from that too, or is that upto the subdomain robot.txt ?