how do i delete a shared admin account

I am being driven mad because I can't delete a shared guest account from my computer. I think it happened when I reset my laptop due with the resetting disc and it held on to the previous account information. I have watched Youtube until I am blue in the face to try and get rid of it and done everything suggested. I have created an new account in order to get rid of the shared but it just won't budge. Does anyone have any suggestions. I don't think it is a Trojan virus as I have a MacCleanse installed and I'm sure it would have picked it up. 😕

Thanks Asp9990