So it sounds like you have this data:
data have;
attrib SSN length=$12 format=$12. informat=$12. ;
input ssn;
cards;
485-40-2594
;
And you did something like this:
data want ;
length SSN $11;
set have;
format ssn;
informat ssn;
run;
To avoid the warning you can just make a new variable instead. That will also mean you don't need to do anything to remove format or informat (as long you you never attach any format or informat to the new variable).
data want ;
set have;
length new $11;
new=ssn;
drop ssn;
rename new=SSN;
run;