Oke, I was wrong about my own code. It's already possible to specify multiple DenyBot options. I just tested it and it worked.
VirtualHost {
...
DenyBot = msnbot:/
DenyBot = yahoo:/
}
How to use the DenyBot option: look for the User-Agent string in your logfile and use a unique work from that line. For example, if a Google bot visits my website, it uses this User-Agent string:
User-Agent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
So, I can block the Google bot for my images and css directory by using
DenyBot = Googlebot:/images,/css