Blocking Idea

Ok,
now there are some hacks out there so i was thinking about this.
i know there is a way to do this.

anywayz mai idea is the special script that we can insert itll then prohibit acess to certain people so if we dont want a certain search engine to find us we can ban them fromcoming on the site that also how people get past registring they replace there adress wth a google bot one this would make the site more secure.