According to the spec, you can only add custom activities.
However, the response's status can have one of these values: ['created', 'updated', 'deleted', 'skipped', 'added', 'removed'].
Personally I've only seen 'added' and 'skipped'.
Is this just a mistake in spec or is it possible to delete them?
This seems like an oversight, but it's a feature that we need added.
The REST API allows us to create custom activities. Humans are fallible, and mistakes will happen. If a custom activity is added to a Lead by mistake (wrong lead id, duplicate send, etc.), there is no way to undo it. That's a problem.
Marketo – please add this feature. Thanks!