data test;
     format date yymmdd10.;
  date='25Oct2010'd;
  put date=;
run;
*** Solution 1;
data remove;
    set test;
format date;
    put date=;
run;

*** Solution 2;
proc datasets library=work nolist;
     modify test;
  format date;
run;
quit;

data _null_; /*just want to check whether the format has been removed, so no need to create a dataset, this is a example for your "data _null_" question*/
     set test;
  put date=;
run;

太感谢le!!