I used to put something along these lines in my configuration file when I used to use an old version of Apache for Windows:
I'm sure the option is similar in modern Apache on Linux..
<Files ~ "*.inc">
Deny from all
However, this won't solve your problem. You can't disable "direct" access to the .css file and still expect it to be usable "within" your site. You can't have it both ways.
Are you trying to protect your css code? You can't. The only way to prevent people from accessing your files is to not make them publically available in the first place