I know when I see things like what you are talking about, I start blocking IP ranges in .htaccess

when I start seeing questionable errors, etc, I check the IP address against various databases to see if they are a known spammer or the like.

I also use a very old script called guardian from xav.com that allows me to add filters, so if someone is probing my site for known hacks and they match my filters, they get hit with a DOS and are automatically locked out of the site. anything that doesn't match an existing condition I get notified about so I can check it out.

"No matter where you go, there you are."
"If you can't do something smart, Do something right"
"There are three kinds of people in the world, those who can count, and those who can't"