In-laws booked plane tickets to visit us without asking, and they’re staying with us.

I would completely make the other bedroom unlivable.

I would start painting it

Take up cheese making

Something that would require the whole room and make it clear that they won't be staying.

Send a text saying

I know you're coming into town. Unfortunately, you can't stay with us.

Here are links to hotels, motels, Airbnb, and campgrounds. Here are links to car rentals. Here are links to local attractions. We hope we can get together for dinner.

Hope you have a blast on your trip.

They want to know why?

Tell them that your spare room is no longer a bedroom and is being otherwise used. In addition, tell them that next time they should ask before assuming that they can stay with you.

Don't apologize

Don't try and accommodate

If you do, that will always be the expectation.