The message you describe is displayed by the grader, which is a separate thing from the answer checker.
Unless you install a different grader for a question, WeBWorK will use the default grader, which divides the grade on the question evenly between all of the answer blanks. Setting showPartialCorrectAnswers to 0 does not change the scoring on the question, it simply suppresses the message to students indicating which parts of the question they got correct. For example, if a student answers one of four parts of a question correctly, they will still receive a grade of 0.25/1 on the question, but they won't know which part they got correct.
If you don't want any partial credit given for a question with multiple parts, then you will have to install the standard problem grader using
install_problem_grader(~~&std_problem_grader);
This will change the question so that there is no partial credit. The student will receive full credit if all answers are correct, and no credit otherwise.
If all you want to do is remove the message, then there is probably a way to do that, but it might involve writing a custom grader.