This will make the pests at PSI (Performance Systems International ) hiccup for a few minutes and they'll come at you with something stronger and perhaps more often.
SetEnvIf Referer [google.com]
Refer based solutions are less than 100% accurate, and may be easily defeated, even if the visitor is not aware that is the reason for denial.
FWIW, raw visitor logs are essential.
If your host doesn't provide them, find another host immediately.
If you've simply failed to turn them on in CP, do so and begin understanding them.