Hi,
You ALLOWED him to continue with you for one more month for completing your project after his resignation?
After resignation and completing notice period by following the general employment conditions, employee deserve to go away and employer need to release him with his dues /F&F respectively.
Well as he has stayed for more one more with you for just completing your project work, firstly, he must have gone away himself because he has taken action or resigned, but as he has not done this, you need to ask him whether he wish to continue with you or want to leave. If he wants to go, you need to release him immediately else he can be reappoint.
Answer of your question in simple word is, you cant remove him from the job as he is already free from employment conditions and now an ex-employee but NEED TO INFORM HIM THAT HE CAN NOW STOP COMING AS THE WORK HAS COMPLETED FOR WHICH HE WAS REQUESTED TO STAY MORE with his F&F statement and dues (and can ask him to come later for his F&F, if required).