I presume he does not have an admin account? I guess you could try 'lock' the particular file that holds the keywords so it cannot be changed. This can be done easily on a mac using file information. For windows, file properties should do the trick I suspect. (could also choose hide.)
What could undo this? If he unlocks it or deletes the preferences folder. That may depend on whether he has admin abilities in his account.

Which file? I'm not 100% sure to be honest. For LW 5 I believe it's probably the Library5.dat file inside the preferences folder.

Will this work? Not sure. I tried it on my virtual windows & the library5.dat file kept unlocking itself.

I am aware there are 3rd party file lockers out there. Some 'might' be free I don't know.

Someone else might have some ideas about your scenario.