1. Whether legally a charge can be made without a service being provided. Probably it can, although even with taxes in theory the tax authority can justify you ARE being provided with a service in return for the tax you pay. However, for a sector outside CAS at night you are NOT being provided with a service (even if you want one).
2. If you didnt pay, who will sue you? I reckon it will be Eurocontrol not NATS or whoever they are agents for, because IT IS Eurocontrol who issue the invoice. Could well be wrong of course.
I guess they would quote all sorts of service, which you may or may not elect to use, but are paid for by the IFR en route charge for the benefit of all airspace users, such as weather data, AIS, en-route navaids, LARS and ATSOCA by NATS units, etc. It's a bit like the UK TV licence I suppose. You pay whether you watch the BBC or not.
If you didn't pay, then they'd probably impound your aircraft any time it landed at a suitable airport where the organisation (i.e. an airport authority or owner) was geared up to do so. You'd be given something called a Mareva Injunction if I remember rightly and moving your aircraft could see you in a whole heap of trouble.