You cannot re-format a DVD unless it is a DVD + - Re-writable. If it is a re-writable DVD your DVD burning software should give you the ability to re-format the disk.
You could try ripping the contents to your hard drive and then create a new DVD using Nero/Sonic etc. - You may need to download a code breaking app if its a commercial DVD - or use DVD shrink.
Google: DVDshrink - it's a free download, and a good one too. Lets you compress 9 gig DVDs down to 4 or less. Worth getting even if it doesn't solve your immediate problem. Best of luck!