A lot depends on what type of lounge and how you obtain access. If this is a "real" lounge, e.g., an "international premium" lounge where you gain access through a paid F/J ticket or status and you purchase a full F ticket so that you can access the F lounge, that is one scenario. Another is purchasing a refundable domestic ticket to get through the check point to access your paid domestic UC or the like lounge just because you can't be bothered to wait at a counter for a gate pass.

There are two issues. One is inventory spoilage, e.g., the theft of services fraud inherent when you purchase a ticket which you do not intend to fly at the time you purchase the ticket and the other is the real cost of lounge access.

This is far from an ethical issue. It is theft/fraud however you want to dress it up and while it is unlikely in the extreme that you will be pursued and prosecuted for it, that does not change the fact that, at least in the US, it is. The fact that you won't likely be prosecuted does not make it not a crime.

But, carriers do care. A lot. And they monitor patterns and spend a small fortune on sophisticated software. Close-in purchases of F tickets which are then cancelled after lounge access on a regular basis are a good sign of fraud. Linked to false names, but CC's in the same name, same IP address and so on, it can be quite hard to build a truly false identity.

If the passenger is a FFP member, the easiest is to simply fire him. While it was not over lounge access, the recent UA thread over the GS who routinely cancelled F tickets at the gate and had his GS status cancelled and his MP account closed is a good cautionary example. UA legitimately determined that someone who routinely cancels at the gate had no intent to fly the ticket in the first place and that the passenger was a fraud. Sue him? Refer him to authorities for prosecution? Heck no. Why bother? Just shut him down.

Not quite fired, but it's a long way from GS to nothing.