You want to create an alert but not to log something.
Use the log_null output plug-in. This allows you to create an alert that doesn't create log entries.
There are occasions when you might want to draw attention to an event, but not record it for future reference. This allows you to create a powerful monitoring system, but not to have to worry about using huge amounts of disk space logging everything.
Snort Users Manual
Installing Snort from Source on Unix
Logging to a File Quickly
How to Build Rules
Detecting Stateless Attacks and Stream Reassembly
Managing Snort Sensors
Generating Statistical Output from Snort Logs
Monitoring Network Performance