check logs
higher than average page requests from one source is suspicious.
Except if they come via gateways/proxies aol bt etc.
you can ban the ip for a while
Check referrers
you can ban by referer and / or by ip and referer better than robotes.txt
If you are realy certain you could build a litle "honey trap" make up a juicy company get a high rate phone number and keep them on hold for as long they would hold.
Alternatively generate a nice page explaining this without mentioning names... just we would never dream of calling..... blah blah...
please inform us immediately.... confirmation e-mail is a good place
just some ideas but make sure you're right first.....
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