Can I erase or otherwise format a CD?

No. After a CD or CD-R has been created it cannot be edited, erased, formatted, or otherwise modified. If you've created the CD or CD-R and want to modify it recreate it using a new CD-R.

If you're using a CD-RW disc it can be modified, but keep in mind that information written to the disc is linear, which means if you have modified a file and need to have it updated on the disc, it needs to be re-written to the disc. The existing file on the disc cannot be modified.