How do you stop spammers from using your address when they spam other people?????
It's not very hard to put someone else's Email in the "reply-to" field of the Emails that you send. & I'm afraid, that there isn't much you can do to stop that.
It's even a technique that spammers use to attack certain companies ( those that fight against spam for example ). You send a bunch of Emails to adresses that you know do not exist & in the "reply-to" field you put an Email adress of a company or someone you don't like. They'll be recieving a bunch of error Emails for the next few hours.
It is not just the reply-to field. Due to the nature of the e-mail protocols you can put any address in the "from" field also as long as you are using a custom mail sender and a stupid SMTP server.
There is nothing you can do to stop it except for going out and shooting the spamming bastard on the spot.