How can I prevent Excel from opening recovered files?
You are probably dealing with an AutoRecover issue. The Office programs will try to reopen documents that are open when the programs crash (a Force Quit is essentially a crash to the programs).
This link explains what's going on and how to recover from the problem. To quote the relevant part:

AutoRecover saves files in the ~:Documents:Microsoft User Data:Office 2008 AutoRecovery folder. These files are hidden, and their file names starts with ~ar.
You can see hidden files if you navigate to the folder above in Finder, type ⌘F, and choose FIle Invisible from the Kind dropdown - if that's not an option, choose Other..., and check the  File Invisible checkbox - and set it to Invisible.

Try making the hidden files visible, then moving them to the Desktop, then start Excel. Of course, if you're using Office 2004 or Office 2011, they will have their own AutoRecovery folders.