I do not believe there is any way at the present time from within Fusion itself that this can be done.
While I haven't tried the following and it may not be practical for you I'll mention that if the Boot Camp partitioned was not a mounted volume while in OS X maybe it wouldn't show in the Virtual Machine Library window. I don't have a Boot Camp partition at the present time to test with and you may also have to edit or delete the "/Users/$/Library/Preferences/com.vmware.fusion.plist" file to remove the existing Boot Camp partition entry from the Virtual Machine Library window afterwards. Then as long as the Boot Camp partition was not mounted in OS X while Fusion was open it would not reappear on the Virtual Machine Library window. As I said I haven't tested this however I think it might work and then again maybe not.
Well I finally had time to test not having the Boot Camp partition being mounted are startup of the Mac via /etc/fstab and it did not stop Fusion from detecting the presence of the Boot Camp partition and it still populated an entry on the Virtual Machine Library window.
So it appears that VMware has hard coded this into Fusion and has left the User not being able to do something piratical like not have the Boot Camp partition in the Virtual Machine Library window if one does not want it to be there and I for one am one of those that would like to have the ability to not have the Boot Camp partition show in the Virtual Machine Library window unless I what it to just like any of the other Virtual Machines I have!
I wounder how hard it would be to have a preference in Fusion's Preferences that would enable/disable automatically detecting the present of a Boot Camp partition and allowing it to be deleted from the Virtual Machine Library window?
I hope someone from VMware get the message.