F5 iRule for FormBased Credentials drop or rewrite

Or I ask my question in other form: How can I remove characters from login password in F5 via iRule to a form based login page from an application, the reason is from outside we want to not give a hint that the sysadmin exists, so if come I from outside with the sysadmin the password should be changed to 000000 so that it is invalid, from inside should the user login be possible with the sysadmin ! I coud do to prevent the access based on the username sysadmin but after I check the username I have to make a decision "drop" or "reject" or "403" whatever.