There are a few ways that this can happen, and which direction depends on how needs their plan covered (primary contact vs plan member) and why they need to cancel their plan (new job vs enrolling in Medicaid or Medicare).

The easiest way is to just call the Marketplace directly at 1-800-318-2596. For some cases however, you can log into your account and manually de-enroll plan members. If you go with this method, make sure you know what you’re doing however; doing things in the wrong order can really mess up coverage if done incorrectly.

This link to healthcare.gov can also walk you through how to cancel based on your situatiuon.