You can approach this in a couple of ways. The most obvious way would be to create a separate user account and set up OmniFocus there, which would allow you to completely separate work and personal data. That's somewhat less than convenient, however. The other approach that comes to mind is to make a second OmniFocus database in the same user account, then remove all of the unwanted data. OmniFocus can have multiple databases open simultaneously, and you can drag and drop from one to another. The catch here is that the alternate database will not be synced if you use OmniFocus on multiple machines, and as perspectives are stored in the main database file, I don't believe there's a way to get them to work with your alternate database.
If you want to try out the alternate database idea, it's easy enough to do. Launch OmniFocus, then select File->Export. Set the export type to OmniFocus database, pick a suitable file name and location, and click Export. To open that database, you'll double-click on the file in the Finder. Windows from the alternate database will have its filename as part of the window title. Delete any projects you don't want in the alternate database, but check the window title first to make sure you are deleting them from the correct database!