Never - EVER (!!!) - reply to "unsubscribe"-adresses of services you haven't subscribed to yourself in the first place. Random junk mail ist usually sent to hundreds of thousands of mail adresses and replying to them only shows the sender that your email-adress is a valid one and you will then begin to receive junk mail from various other sources, as your adress will be traded in mailing lists...
Depending on your ISP/mail host you might wand to use some spam filter or a program called
Sendmail.
The first will put all unwanted spam mail into a seperate folder. The second will prevent delivery of said spam mail and therefore report your email-adress as invalid to the sender. Doesn't work for all providers, though.
See also
this posting for info.