You can’t do that…it is not possible…that is the answer!

Most web servers have a FTP (File transfer Protocol) where users and webmasters can upload parts of there site like .html or .php web pages. Getting through this requires a username and password only the owner of the website has. No, FTP and Webservers are not inpenitrable, but judging by your question you probably dont have the expertise to get around security AKA Hacking. (Not to mention its illegal)

Hm.
1. Attack servers
2. Hack into servers
3. Site has illegal material – then you can report it and it will be taken offline. However for this to be done you need substantial proof.

Servers is what is used to host the site. If you attack them, you will likely get caught, sued + possibly serve jail time.
If you hack, the same will be done.

Simple answer- You can’t.

Porn and phising sites aren’t illegal.OK if you use phisin to steal information it’s illegal but still…
Anyone can make a site and upload anyhting he likes (as long as it’s legal). You can’t just take that away because you like it that way. Nevermind the fact that doing this is in factillegal.
Anyway, even if you could do something like that it wouldn’t change anything, new ones would be uploaded in a heartbeat.
To answer your question you can delete the contents of the site but there’s no way you can not do this and not get traced…