My employer also allows pre-taxed payroll deductions for AFLAC or other insurance companies. I have an AFLAC policy myself (a cancer policy) and the pre-taxed premium payments are deducted from my bi-weekly paychecks.
Through my own experience I know that once you sign up for a policy with AFLAC, you also have to fill out a payroll deduction authorization form telling your company's HR department that you want $X deducted from your paycheck each payroll period. Perhaps what happened here is that you informed AFLAC that you changed your mind, but the paperwork was already in your company's HR department so they didn't know you changed your mind and made the payments like the paperwork stated. Had you informed HR at the same time you informed AFLAC, all this may have been avoided.
Also, my employer has what is called "open enrollment" periods every year. During this time, we can look at the insurance policies we have and make changes - sign up for new policies, drop current policies. I think this is pretty standard policy so it is important to review all of this information very carefully so that you know exactly what you're paying for out of your paycheck.
Also, as others have stated, it's also important to check your paystubs because you never know when a human error or a computer error may occur. It's good to catch those mistakes early.
So, in closing, I don't think AFLAC is to blame. Your company sent in the premium each month, so as far as they were concerned you took out the policy and paid for it. And I don't think your HR Department/Accountant/whatever is to blame either if they were not notified that you had changed your mind about the payroll deduction - they were going by the forms you signed and submitted to them.
It seems to be an oversight on your part, plain and simple.