despite ticking the box "remove all website data", when I reset Safari not all stored cookies are eliminated. How can I overcome the problem? Or is it a bug?

Not a bug, just a species of nasty persistent cookies.

http://samy.pl/evercookie/

Flash Locally Stored Objects (Flash cookies) can be stopped if you have the latest version of Flash installed.

There's a Flash preference pane in System Preferences > Other.

Set to block all sites from storing information.

That eleminates one source; the rest are more of a pain to find and eliminate.

I found a old work around

Go to Home/Library/Cookies use get info on Cookies.plist file and check the locked box. Now no more cookies should be added

cbdave wrote:

I found a old work around

Go to Home/Library/Cookies use get info on Cookies.plist file and check the locked box. Now no more cookies should be added

Not much of a workaround. Cookies are needed for many sites to function properly.

But not all of the ones they set, by a long way.

Some further reading; http://ashkansoltani.org/docs/respawn_redux.html

The Internet is a jungle!