Unfortunately there isn't any way for direct deleting. I am going to show you a way through the registry. The example shown is for word but you can do the same for excel or power point.
Press start, type 'regedit' end press enter.
Follow the path: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Word\File MRU
On the right you can see some key values. Expand data so you can see all the details. Remove the objects you do not want by right clicking the item that matches to the path of the file and select delete.
By Vasilis Nicolaou