Use OpenDNS (http://www.opendns.com/) as dns server for your proxy: it has automatic block & detection of botnet and similar problems.
Probably you have some kind of infection on the pc we're talking about.
With a free account you'd also get the chance to do a nice web filtering and stats work.
Using malwarebyte's antimalware (http://www.malwarebytes.org/mbam.php) + superantipsyware (http://www.superantispyware.com/download.html) solved all my similar problems and both have free version.
If, this could be the other case, the user is aware of trying a ddos attack just talk with him first and then with the boss.
Nothing works better than that.
As for the 5 requests per minute: That
would make it completely unusable.
I do totally agree, @Hyppy. :)