You can erase a CD-R or DVD-R
The chemical and physical properties of CD-R, DVD + determine the limits meaning that the burning process is not reversible. Once the data recorded, it can't not erase.
But you can actually burn an additional session in which we virtually deletes files.
But this does not free space on the CD-R: The file just disappears from the list of files, but it is still physically present on the disc.
Only the media that has the initials RW are truly erasable, such as CD-RW, DVD + RW and DVD-RW.