To delete a single contact, you can open up their profile, scroll to the bottom and click on the Delete button.
To delete multiple contacts (for example, everyone with a tag):
- Go to the Contacts section, search / select the tag
- Check the ALL checkbox above the names
- Go to the ... more menu on top
- Find Delete (you can delete up to 10,000 contacts at once; for request larger than 10k, please email firstname.lastname@example.org)