Hit the delete key. Responding to a spam is generally futile as most of the return addresses are either forged or invalid. Do not buy products advertisted via spam. Remember that the cost of sending you spam is so low that even if only 1 in 1000 recipients respond to the ad it's still worth it to the spammer. Protect your email address.
Spammers gather email addresses through several methods.
You can use various tactics to prevent this including using a CGI script to handle email from your webpage. This spam stuff is wasting my time and pissing me off. Install email filters. Filtering spam from email can accomplished in various ways. The problem, of course, is that there is an initial investment in time to understand how these filters work and to configure them properly.