You should be able to delete old saves from the save window (select an existing save, and you should be able to overwrite it or delete it).
For the PC version you can manually delete the saves in Windows Explorer, or move them to a subfolder (the game will ignore any new folders you create).
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).