@Alvaro Martínez Peri Thank you for reaching out to us, As I understand you are looking for an option to disable servers from getting onboarded to defender for cloud.
Navigate to this section, within defender for cloud portal, toggle the on/off option for servers.
As a troubleshooting step did you perform clean boot on one of the servers having high cpu ulilization to check if any 3rd software is causing this issue/server is fully patched ?
Let me know if you have any further questions, feel free to post back.