PDFDoc doc = m_viewer.GetDoc();
doc.Lock(); // write lock
Annot annot = m_viewer.GetAnnotAt(x,y);
Page page(m_viewer.GetPageNumberAt(x, y);
page.AnnotRemove(annot);
doc.Unlock();
m_viewer.Update(); // there might be a version of Update available that takes an annot, therefore avoiding refreshing everything...