Do you have an event action override installed on the system to generate an alert for a risk rating (RR) greater than some value? If so, then even signatures that are set to "no action" will get the override applied if their resultant RR satifies the override criteria.
If this is the case, then you have several options...you can adjust the override to raise the minimum RR value that triggers the override, or, you can tune the signature to lower its effect RR. The later can be accomplished by lowering either its Severity level (info, low, medium, high etc) or lowering its Fidelity value.
The signature helps address some covert channels used by some exploit software.
