I don’t think there is any legal way of rejecting a CTB claim as contrived.
The definition of a non-dependant in CTB Reg 3 includes the puzzling para (3), which includes people whose liability to make payments (of rent, I think, in the context of the full text) is contrived, and people who used to be non-dependants before they assumed joint liability for Council Tax.
Trouble is, I don’t see how Reg 3(3) can act to disqualify a claimant from CTB. I think the best it can achieve is to make somebody else a non-dependant in the claimant’s CTB calculation, when it might have been more advantageous for them not to be a non-dep (like when the maximum non-dep deduction is more than half the tax payable, for example). Reg 3(1) presupposes that you already have a claimant, the definition is supposed to apply to other household members. You could argue that para (3) is extra and not dependant on para (1), but if you try to make the claimant a non-dep under Reg 3(3), I think you run into difficulties.
Suppose the claimant is a non-dependant as defined by Reg 3(3); so what? How does it make her not entitled to CTB? She still fulfills all the conditions of CTB entitlement. Reg 3(3) would be irrelevant because it does not affect any of the other entitlement conditions. In particular, being a person described in Reg 3(3) does not make the claimant a “prescribed person” not entitled to CTB under s 131(3)(b) of the Conts and Bens Act. This problem used to affect the old CTB temporary absence Regs, as noted for menay years in the CPAG commentary and by Commissioner Fellner in CH/2111/2003. It took new Regs to bring prolonged absences under s131(3)(b), and at the moment I think it would also take new Regs to disqualify contrived CTB liabilities.
So I reckon you have to pay!