Normally DPlot will prompt you before exiting to ask whether you want to save the current plot if it has been edited in any way. If you find this feature to be annoying, select the General command on the Options menu, and uncheck the Prompt to save changed plots box.
Page url:
https://www.dplot.com/help/index.htm?howdoigetridoftheprompttosavechangedplot.htm