By mistake, I added a session to a CD/ROM.
Since the CD is not mine, I want to try to delete such new session. I know it is impossible to remove files from a CD: I want to make that wrong session physically unreadable. And reactivate the first session.
Otherwise there is enough room on the CD to add the same data of the first session.
How can I do?

Well, you can't delete the new session.  But you're on track with the idea of going back a session and getting the data.  Here is a link to IsoBuster.  I think the free version should do what you need, else you'll need to fork out $26 bucks... but the program is well worth it.

(in reply to ffabian65)

Thanks for your answer; the software you suggest doesn't resolve my problem because I have no problem in reading the old session (Nero Ver. 5 does it). I want to make unreadable the new session.

(in reply to SithTracy)

You can't... I thought you were on to the solution... that being to extract the previous session with IsoBuster and then writing it to a new session...  Your mistake will remain present on the disc... unless you create a new disc and re-burn all of the correctly extracted session in order to the new blank.

(in reply to ffabian65)