No, annotations on a call cannot be deleted by you or by Ringba.
When you create annotations on a call, the system converts the annotations to tags. You can see the tags on the Tags tab of the call details on the Reporting screen. By policy, Ringba does not change tags on a call, so annotations are also permanent.
If you have added incorrect information in an annotation, you can add another annotation with the correct information. Just make sure to include information that makes it clear that this is the updated information.