If you don't use the ticket and don't notify the airline, the ticket simply has no value. If you call them and tell them before flight time that you won't make the flight, the remaining value of the ticket can be used on the same leg of the trip in the future, less the $150 change fee, for up to a year.

If the ticket was inexpensive, you can simply not take the flight. If it is an expensive ticket, you may want to call them and let them know. That gives you the option of using the ticket in the future. If you don't use it within a year, it expires.

Either way, you won't be charged anything extra.

The only way I can see you having a problem is if you do this repeatedly and intentionally because a round-trip ticket is cheaper than a 1-way ticket. And even then, it would have to be on a very large scale for anyone to care, if at all.