I think you bought your test server recently, may be that's why this is happening.
Answer to Question:1
Since the IP address is scarce resource on the internet, it is very likely that the previous owner of your current IP address was somehow using/allowing requests from ads.yahoo.com, images.google.com or whatever the requests you are getting in your logs.
Answer to Question:2
Although its not a great solution, but it will help you to certain extent:
- Write all the IP address to a file(better if you do it pragmatically), while you do this, don't send any genuine request to server.
- Write shell script to read this file line by line and execute iptables -I INPUT -s $FILE_LINE -j DROP (You will need root access)
Basically this command will block these IP addresses by making entry in iptable of kernel.