Unless you're using a CD/DVD-RW type disc, the only way to "alter" what's on the disc is to but a brand-new file on the disc that has an identical name & location in the directory structure as the original. In other words, the only way to "overwrite" a file called "MyData.docx" located in the "Data\Mine" directory on your DVD-ROM would be to create a brand-new file on your PC with the same name, put different data in the file, then tell your PC to burn it to the "Data\Mine" directory of your CD/DVD-ROM.

Now, if you mean that you don't want anyone adding additional data to the disc, I don't know if Windows 7 has that capability or not. However, there are plenty of other programs out there you can use that will provide that feature for you, as well as other features. Nero comes to mind, for example.