Launch StickyBrain. Select "Sticky Browser" from the Control menu. In the Sticky Browser there's a button called Show, and the default is All Types. Click on this button and choose Alarms. What is happening is you have set an alarm for a sticky that won't turn off until you dismiss it. When you choose Alarms, the Sticky Browser will list all stickies that have alarms assigned to them. Simply disable the alarm for the offending sticky note.