I can not understand why you can not give him a second chance? If suddenly the situation recurs (although I am sure that this will not happen anymore) you will block for ever After all, 1 post was published. As stated above, the password will be changed, and it will be more responsible in the future. Yes, I know English badly, because I translate you with the help of a translator.
Have a look through the last few pages of the Meta board. You will find several posts on each page from people complaining they were banned following an instance of plagiarism. None of them were given a second chance. Why should you be any different? Your excuse that a hacker broke in to your account, made a single post and then never returned is obviously false.
Rules are rules. If you break them, there are consequences.