Sometimes, when you post your email in a public forum or webpage web-crawlers collect it and adds it a mailing list. Typically, it is sold to a company or used by a company to send malicious emails, so when you are divulging your email over the web you could disguise it a little (for e.g. firstname (no space) lastname AT huge (no space) emailproviderDOTcom.

There is nothing you can do now except unsubscribe from them as they come in.