QGsMapToolのcanvasReleaseEventイベントで実行
[code]
def canvasReleaseEvent(self, e):
marker = QgsVertexMarker(self.canvas())
trans = self.canvas().getCoordinateTransform()
p = trans.toMapCoordinates(e.pos().x(), e.pos().y())
marker.setCenter(p)
marker.setColor(QColor(255, 0, 0))
marker.setIconType(QgsVertexMarker.ICON_CIRCLE)
marker.setIconSize(20)
marker.setPenWidth(5)
[/code]

marker.hide()で非表示
marker.show()で表示