Below is sample code that illustrates how to specify a white filled marker with a black outline so the line connecting the points is not seen through the markers:
proc sgplot data=sashelp.class;
series x=name y=age / markers markerattrs=( symbol=circlefilled color=white) markeroutlineattrs=(color=black) filledoutlinedmarkers ;
run;

Regards,

Lelia