First search for open libreoffice files:

ps aux | grep libre

For example the output of mine is:

hadi  21426  0.1  0.0 205328  3468 ?        Sl   14:17   0:00 /usr/lib/libreoffice/program/oosplash --writer

hadi  21445  9.8  0.7 1269272 179872 ?      Sl   14:17   0:01 /usr/lib/libreoffice/program/soffice.bin --writer --splash-pipe=6

then

sudo kill -9 ID

the ID is the second number for (soffice.bin) not for oosplash

so in my example:

sudo kill -9 21445

You need professional Ok:

ps aux | grep -i office | awk {'print $2'} | xargs kill -9

hope this is professional in your evaluation!!

or more minified command

kill -9 `pgrep -lf soffice.bin | awk {'print $1'}`

or more minified minified minified command

pkill soffice.bin

EDIT:

All libreoffice open files take the same PID, for that you can't just kill writer and keep impess for example.

And to prove my point of view the recovery tool in office is unique for all files. What i mean if you close a writer in imporper way and then open an impress for example then impress will request you to recovery the writer file and it did so and this proves my answer