I guess the only way is a registry hack - these are files that the software has previously worked with. They will eventually "go away" as you work with more "real" files.
DON'T go hacking the registry on your PC unless you know what you are doing, I take no responsibility for your actions, but if you want to get rid of the MRU (Most Recently Used) file list, they are stored here in the registry....
My Computer\HKEY_CURRENT_USER\Software\Rockwell Software\RSLogix500\Recent File List
It appears to be fixed at 8 entries, I can find no setting for number to remember.
IMHO I wouldn't worry about the list, the most recently used file will always be number 1 in the list.