These days common problem of hacking of website by hacker with scrips. They always use some common method or string to hack it. Now you can prevent these requests by adding some simple .htaccess rules.
You could send a 403 Forbidden
(access denied) in case -admin
is requested :
RewriteEngine On RewriteCond %{REQUEST_URI} ^/(.*?)admin RewriteRule ^(.*)$ - [F,L]