public bool Cancel { get; set; }

Public Property Cancel As Boolean

void chartControl1_AnnotationRepositoryChanging(object sender, AnnotationRepositoryChangingEventArgs e) {
    if (e.Annotation.Name == "Annotation1" && e.Change == AnnotationRepositoryChange.Deletion) {
        e.Cancel = true;
    }
}

Private Sub chartControl1_AnnotationRepositoryChanging(ByVal sender As Object, ByVal e As AnnotationRepositoryChangingEventArgs)
    If e.Annotation.Name = "Annotation 1" AndAlso e.Change = AnnotationRepositoryChange.Deletion Then
        e.Cancel = True
    End If
End Sub