If you're not interested in receiving offers to sponsor your Meetup Group, just click 'Sponsors' in the top navigation bar on your Meetup Group's home page. Uncheck the boxes for Perks and click 'Save'.
That's it! Your Meetup Group will no longer receive sponsorship offers via email.
Important note: opting out of Sponsorship offers is a Group setting. If you are the Organizer or Co-Organizer of a Meetup Group, and you opt out, you aren't just turning the emails off for yourself: you are opting the Group out of receiving offers.