If you feel lucky, you can delete the entries from your INI file.
Look for a section like this:
[Recent File List]Keep the
File1=C:\Program Files\Revit Architecture 2009\Program\SDK\VSTA Samples\Revit_VSTA_Samples.rvt
File2=C:\Projects\0810500 Davisco\Davisco - Disco.rvt
[Recent File List]
part, but you can delete all of the File#=blahblahblah lines
Again, this only works for the current session. as soon as you open another project, it will get added to the list. But at least it will be the only one in the list.