Well the bad news is you can't stop it, it is fairly easy to spoof the email address. And you can't effectively stop others from using your email.
The good news for you is that most of the time your spam folder is going to filter this out due to the spam like nature as well as the other identifying factors. Emails have some information in the headers that usually will be a sign that the email is spoofed thus alerting your spam detection system. Since gmail is picking it up as spam and NOT your account then whoever receives that type of email will be placed in the spam folder while your legit emails should be fine.
Bad News - Can't stop them effectively
Good News - Hard to really spoof an email the correct way and change and alter each bit of information correctly, and spam detectors are getting better and better