A proper delete function would be great. As a workaround, we are scrambling deleted contacts' email addresses ourselves via the PUT API call for updating a contact:
PUT https://api.maropost.com/accounts/####/contacts/<contact ID>.json
with a body of { "contact" : { "email" : "<anonymized email>"} }
Don't forget to also remove the contact from lists via the existing delete_all endpoint as well.