Hi 
Yes, the easiest way to deal with this sort of thing is to shut-down the machine. Then switch it on again. Before opening LibreOffice again delete the offending file. When you do open LibreOffice it might ask if you want to recover the file. Just click on the "Cancel" button to open LibreOffice normally.
There does seem to be some process running or the machine thinks there is a process running. In most Operating Systems you can find a task manager and use that to "kill" the process. Most seem to give you a task manager if you press
Ctrl Alt Del
but sometimes that forces the machine to reboot so it's worth saving your position in all open documents and similar.
In Gnu&Linux you could try this on the command-line
ps aux | grep soffice
ps aux gives you the equivalent of the task manager as a list in the "terminal console" / command-line window. The | is either near the Enter key or near the left-hand shift key. It looks a bit like a straight vertical line with a small gap about half-way down. We call it a "pipe" and it allows the initial command to have extra things done to it before giving us an output. In this case we "grep" (which is approximately saying "search for"). Hopefully you get 2 lines, roughly like this
user 7341 5.6 4.9 1274940 100512 ? Sl 16:03 0:01 /opt/libreoffice4.0/program/soffice.bin --writer
user 7362 0.0 0.0 13588 872 pts/0 S+ 16:04 0:00 grep --color=auto soffice
That 4 digit number might be 5 or even 6 digits. It is the "process id number". You can force the process to stop by using the command
kill 7341
but just replace my number with your one. Note the process shown by the line that has "grep" in it has already finished so if you kill the wrong number it just says "no such process currently running". In which case you can try the other number.
Bsd and therefore Mac might have the same commands but it might be a little bit different.
I think all 4 platforms have some nice gui called "task manager" that you could use instead. I just don't know how to get it in Mac or other Bsds.
Anyway, one certain way to end the process is to shut-down the machine and then switch it on again. Similarly if you ever need to remove a Usb-stick then you could just shut-down the machine to make it safe to remove instead of doing all the fiddling around.
Regards from
Tom 