I find it worrying that Captains do not exercise their authority in such cases and either cave in to commercial pressure for illegal (unrestrained) carriage, adverse publicity or worse, blame Ops for a "decision" that is not for Ops to make, slope shoulders and do as they think the company wants.
Whatever happened to Captaincy?
If the unprofessionalism of all the staff they've encountered up to that point allows someone too big or too unwashed with regard to the comfort and safety of other pax to even reach the aircraft then it is ultimately the Captain who must make the decision and "adverse publicity' should not be a matter that even remotely affects such a decision. It's no different to allowing drunks to fly. People who can't fit in seats or be safely restrained need to either buy and occupy multiple seats or simply not travel.