First of all, your getting your terminology confused. a DVD-ROM is unwritable, these are the disks that movies etc. come on.
As you said that you have written to it, this means that you either have a DVD-R (writable once, then it becomes like a DVD-ROM) or a DVD-RW which can be written to multiple times.
To answer your question, it depends whether it is a DVD-R or a DVD-RW. Normally it says somewhere on the disk, if its a DVD-R then you can't erase it I'm afraid. If however it is a DVD-RW (or your not sure) then pop it into your PC drive, and go Start>Computer, right click on the disk and look for an option that says "Erase Disk" if it's there then you can erase it, if not then I'm afraid it can't be done.