If you want to erase a medium, the easiest way is to choose Recorder->Erase from the main menu or just click the Erase rewritable medium icon on the toolbar.
Choose the drive, speed and hit the Quick or Full erase button. If you aren't sure which option to choose, please read the explanation below.
As the name already says, quick erase is very fast (about 1 minute) in contrast to full erase, that can take more than half an hour. The difference is, that you only delete the table of contents (TOC) if you choose quick erase. The data stays on the disc, but the PC can't do anything with it. This is the recommended option. However, if you have sensitive data on your medium, you should choose full erase, because then the disc will be completely emptied.
As background: with full erase the burner "melts" a special part of the CD's material, that becomes solid after a very short time and so there is an absolutely blank CD left.