Per @guillaume blaquiere and @John Hanley:
You can get the IAM policy on a project, look for the account you want to remove and set the IAM policies without this account. If there isn't client library for that, use the discovery API or directly the Rest API.
An IAM Identity (principal) is not a Google Cloud resource type. You must use the APIs for each resource type (project, storage, compute, etc.) if you wish to manage the IAM bindings for an identity. IAM Bindings for a project are part of the Resource Manager API. Example python code. However, IAM roles can be assigned at the PROJECT and at individual resources such as a KMS Key, Cloud Storage object, etc. If an identity has an IAM role binding at both the project and a resource (e.g. Cloud Storage object), removing the role binding at the project will not remove the role at the object resource level.
Note: A service account identity is an exception. It can be both an identity and a resource. Usage as an identity and as a resource is managed separately. When used as a resource, it supports IAM bindings, just like a project.