A variety of marker types (i.e., circle shape, star shape, etc.) are available in the Markers toolbar in 2D.
While you are in Markers mode, the tracing functionality is disabled. To return to tracing, click the marker type that is highlighted in the Markers toolbar; this will deselect the marker type and turn off the Markers mode.
To delete the last marker placed, press CTRL-Z
To delete all markers of the same marker type, right-click over the marker type in the Markers toolbar and select Delete All Markers of This Type.
See About markers