It’s not possible to disable your crypto key itself. Instead, disable URLs for the Client ID in the Support Portal > Manage Client ID page. This is a task that cannot be done in the Google Cloud Platform console.
More on Digital Signature
Google generates a single crypto key for digital signing per plan and it is provided to customers at account inception time in the welcome email. They only provide one and offer no tooling for managing the lifecycle of the crypto key nor generating new keys. In short, Google wants each customer to have one and only one crypto key.