You have set the "Usage of the maximum configured disk space for Spam Quarantine" alert as checked under Administration - Alerts - Disk space, and the "Maximum size of Quarantine" setting as not checked under Spam - Quarantine Settings. However, an alert is still sent when the usage exceeds the number displayed for the Maximum size of Quarantine even though not checked.

This is expected behavior. In order to stop this alert you would need to uncheck "Usage of the maximum configured disk space for Spam Quarantine" under the Alerts page.