I figured out how to do it on my own. For anyone else who's looking for the same thing, here's how I did it:
1. Log into your account at xfinity.com
2. Click into the Settings tab
3. In the left sidebar, click "Communications & Ad Preferences"
4. Under Marketing Preferences, click Manage Permissions next to "Sales Calls, Mail Advertising and In-Person Visits"
5. Fill and submit the form that appears
Also, you can opt out of Comcast solicitations here: https://pc2.mypreferences.com/Comcast/OptOut/Default.aspx and you can opt out of Comcast email solicitations here: https://pc2.mypreferences.com/Comcast/OptDown/opt-down