Can you remove a CRM from Marketo?
You cannot remove a CRM integration from an instance. Once the CRM of your choice is clicked (Dynamics or Salesforce) the instance is set to use that CRM for the remainder of its life.
Once the login and mappings have been setup for a instance, the CRM instance or org cannot be changed as well.
If for a reason you do need to change the CRM or the linked instance, you would want to reach out to your Customer Success Manager to get information on provisioning a new Marketo instance.