Two words “potential” and “reasonable.” One you used and the other you didn’t.
I am afraid that “potential” isn’t a reason for exclusion. Every day most travellers (and others) are subject to checks, restrictions and a host of other inconveniences on the basis that they (and most everybody else) have the “potential” for causing harm or disruption. Everyday in our line of work we brief and prepare for “potential” problems and scenarios. In most cases that proves to be an excess of caution but it is “reasonable.” You could take things further and say I refuse to continue. Not because of the fact but because of the ”potential.” In most cases that would prove to be unreasonable.
Having the authority to take “ANY” measures (a word you have emphasised by capitalisation,) would seem to suggest that you think this negates the requirement for those measures to be reasonable? Whether you are standing before a judge or your chief pilot, it would be incumbent upon you to show your actions were reasonable in the circumstances.
There are many high profile “celebrities” who over the years have been reported, arrested or convicted for their behaviour on airplanes. Those incidents (in some occurrences) are matters for the police and the justice system to deal with. Beyond that, it isn’t for me to decide whether they fly or not. Your employer has presumably made the informed decision to enter into a contract of carriage with the passenger. The passenger (whatever their past history of alleged transgressions) has similarly contracted to behave in accordance with those conditions of carriage. Unless there is some present and real concern I can’t really see what your “reasonable” involvement is?
As we all know, crew and ground agents and other passengers will sometimes raise a concern (as they properly should) if a passengers demonstrated behaviour is giving rise to a problem, and it isn’t unusual in those circumstances for a Captain to be asked to make a decision on acceptance or exclusion as the Captain reasonably sees fit. The Captain does have the authority and in some cases the obligation to exclude. However, that authority doesn’t absolve the Captain from having to justify the decision subsequently, nor does it protect him or her from any consequences that might arise as a result of the improper exercise of that authority. Part of the reason why you get paid the “big bucks!”
In your case (and purely based on what you have said) you said that you were “not biased about him (the passenger) or whatever” but in your opening remarks you talk about his social media following and his perceived or reported history. It seems as if your decision was indeed based on your bias rather than any imminent threat or real cause for concern. It appears that your employer was happy to contract with the passenger and (beyond your bias) the passenger wasn’t reported or presenting as a problem. Based purely on that, I suspect you are going to have some difficulty in showing that your exercise of that particular authority was reasonable!
In my opinion it wouldn’t have been unreasonable to raise your concerns (bias notwithstanding) with your crew by way of briefing or informally to highlight that they should be aware and not hesitate to raise any concerns on this matter if they should actually arise. I think what you did (again, based purely on your account) was an overreach of your authority and a mistake. We all make them and learn from them and I hope your management is comprised of people who also realise that. Given the forum this is in I know only too well that is far from being the case in many companies. Good luck.