Block images, CSS or JS if the user did not visit my site

I suddenly get a lot of requests to my site (wget, curl etc...).

.htaccess or PHP protection code against multiple speedy requests

I am looking for ideas for how I can stop external scripts connecting with my site. I'm looking for the same kind of idea behind Google. As in if a certain amount of requests are made per a certain amount of time then block the IP address or something. I thought there maybe a htaccess solution if not, I will write a PHP one.