You have multiple options here.
1. You can check the referrer of the visitor and if its empty, you can redirect them anywhere else.
2. You can allow the direct visitors to view your site, but have Analytics to not track them by wrapping the analytics code in a function.
3. You can check for the useragents to see if they represent a real browser.(well, can be easily faked though.)
4.You can redirect / block javascript disabled visitors.
5. Or just dont worry.
Lot of options to choose from.