How to configure .htaccess ? Deny public access to current/special directory and below ?

I want to deny/disallow the public access to the directory

....\public_html\aaa\

and its sub directories e.g

....\public_html\aaa\bbb\

As far as I know this can be setup by editing the .htaccess file in the directory

....\public_html\aaa\

How do I do this? What do I have to enter to achieve the restrictions?

Is there one single .htaccess file for the whole directory tree (beginning with

....\public_html\

or can I put .htaccess files with higher priority into each sub directory?

My current (default) .htaccess file is (assuming testtest.com is my domain):

---
IndexIgnore .htaccess */.??* *~ *# */HEADER* */README* */_vti*

<Limit GET POST>
order deny,allow
deny from all
allow from all
</Limit>
<Limit PUT DELETE>
order deny,allow
deny from all
</Limit>
AuthName www.testtest.com
AuthUserFile /home/testtest/public_html/_vti_pvt/service.pwd
AuthGroupFile /home/testtest/public_html/_vti_pvt/service.grp
---

Peter