public void ValidationUserInfo(string userName,string password)
{
    //Do validations with password, after converting it to secureString and fetch it out?....

    //Just clear the memory!
    ClearSensiveString(password);

    //Do any others or return after validations.....
}

//Your memory clear way:

static unsafe void ClearSensiveString(string sensiveValue,char coverChar='')
        {
            fixed(char* c = sensiveValue)
            {
                for (int i = 0; i < sensiveValue.Length; i++)
                {
                    c[i] = coverChar;
                }
            }
        }