If you buy a round-trip and only fly one-way there are no legal actions the airline can take against you.

However, they COULD freeze your frequent flyer mile account, but such actions are rare for first-time offenders.

You are best advised to simply call up reservations and advise them you won't be flying due to "unforseen circumstances" and that you won't be rebooking at that time.

Buy the cheapest round-trip you can, which usually includes a Saturday-night stay.