To prevent developers from creating user-managed service account keys and reduce the risk of key mismanagement, you should enable an organization policy that specifically prohibits the creation of these keys.

Enable an organization policy to prevent service account keys from being created (C):

Google Cloud provides the capability to enforce organizational policies that restrict various actions, including the creation of service account keys. By enabling this policy, you ensure that developers cannot create new user-managed service account keys, thus minimizing the risk of key mismanagement and potential security breaches.

References

Service Accounts documentation

Organization Policy Service documentation