You can either turn off the geofence entry alert or geofence exit alert, but not both. One of the alert conditions must be active for a geofence.
You can reduce the frequency of geofence alerts by selecting the A summary of reports once per hour option under Send emails. This will prevent you from getting alert emails every time a geofence event takes place. Instead, it provides you with a summarized report of all the geofence events that took place in the last one hour.