It is not possible to change the severity for firing incidents in CS-MARS as it is a calculated value based on details specific to the incident. If you are not wanting to receive IPS alerts for a specific network behavior, you may want to look into creating an event action filter (EAF) on the IPS sensor to remove the produce alert action (device-side tuning) or create a drop rule within CS-MARS to only log the event to the CS-MARS database and not generate an incident (appliance-side tuning).
Scott