If you are responding to spam mails in an attempt to stop them, you are informing the sender that someone is looking at the mails. The best option is to block the sender if the mails are repeated and stick the mail in your spam folder. There is no point in blocking mails that will only arrive one time. Adding mails to the spam folder informs your ISP that it is spam. Some ISPs are proactive with spam accounts and block them at the server.