What do you mean by undo a partition?
Lets clarify here. Parallels is not capable of creating a real partition on your HD. It creates "disk images", actual files on your computer. These can easily be deleted should you choose to do so.
If you created a BootCamp partition, the removal process is as easy as navigating to Applications > Utilities > BootCamp Utility and using the wizard to restore your HD to a single partition.