Perform the following in the Azure Console:

If data stored in the disk is no longer useful, refer to Azure documentation to delete
unattached data disks at:

1. https://docs.microsoft.com/en-us/rest/api/compute/disks/delete
2. https://docs.microsoft.com/en-us/cli/azure/disk?view=azure-cli-latest#az-disk-delete

If data stored in the disk is important, To encrypt the disk refer azure documentation at:

1. https://docs.microsoft.com/en-us/rest/api/compute/disks/update#encryptionsettings
2. https://docs.microsoft.com/en-us/cli/azure/disk?view=azure-cli-latest#az-disk-update

References:

1. https://docs.microsoft.com/en-us/azure/security/azure-security-disk-encryption
2. https://docs.microsoft.com/en-us/azure/security-center/security-center-disk-encryption?toc=%2fazure%2fsecurity%2ftoc.json