Then it's probable that one of your objects still has the behavior attached to it.
This is a little tricky to determine but if you know what the name of the plugin is/was when you added it (so for instance, when you add a normal sprite object, you may give it the name Herosprite), then you can search the event sheet (in the Event Sheets folder of your project) for any occurrence of the plugin name.
This is only worth doing if you have lots of objects. If your project only has a few objects, then you may as well just check the Properties window of each one to see if the plugin behavior is still attached to any of the.
If your vision so exceeds your ability, then look to something closer.