Further to geno888's question,
Unless the disc was written using packet writing software which simulates a hdd where files can be deleted and added one at a time, you can only erase the whole disc using a program like dvdinfopro or your favorite burning program like Nero. You will be presented with 2 options, quick erase or full erase. Quick erase takes a minute or less and is usually fine unless you absolutely want to fully remove the files from the disc. Full erase can take considerably longer as high as 1/2 hour to an hour depending on the disc rating and your burner.