Why not have a look at the conditions of carriage of the airline concerned?
Here are the ones of a well known British airline:
7a) Our right to refuse to carry you
We may decide to refuse to carry you or your baggage if one or more of the following has happened or we reasonably believe may happen.
7a1) If carrying you or your baggage may put the safety of the aircraft or the safety or health of any person in the aircraft in danger.
7a2) If carrying you or your baggage may affect the comfort of any person in the aircraft.
7a3) If you are drunk or under the influence of drink or drugs.
7a4) If you are, or we reasonably believe you are, in unlawful possession of drugs.
7a5) If your mental or physical state or health is a danger or risk to you, the aircraft or any person in it.
7a6) If you have refused to allow a security check to be carried out on you or your baggage.
7a7) If you have not obeyed the instructions of our ground staff or a member of the crew of the aircraft relating to safety or security.
7a8) If you have used threatening, abusive or insulting words towards our ground staff or another passenger or a member of the crew of the aircraft.
7a9) If you have behaved in a threatening, abusive, insulting or disorderly way towards a member of our ground staff or a member of the crew of the aircraft.
7a10) If you have deliberately interfered with a member of the crew of the aircraft carrying out their duties.
7a11) If you have put the safety of either the aircraft or any person in it in danger.
7a12) If you have made a hoax bomb or other security threat.
7a13) If you have committed a criminal offence during the check-in or boarding processes or on board the aircraft.
7a14) If you have not, or do not appear to have, valid travel documents.
7a15) If you try to enter a country for which your travel documents are not valid.
7a16) If the immigration authority for the country you are travelling to, or for a country in which you have a stopover, has told us (either orally or in writing) that it has decided not to allow you to enter that country, even if you have, or appear to have, valid travel documents.
7a17) If you destroy your travel documents during the flight.
7a18) If you have refused to allow us to photocopy your travel documents.
7a19) If you have refused to give your travel documents to a member of the crew of the aircraft, when we have asked you to do so.
7a20) If you ask the relevant government authorities for permission to enter a country in which you have landed as a transit passenger.
7a21) If carrying you would break government laws, regulations, or orders.
7a22) If you have refused or failed to give us information which a government authority has asked us to provide about you, including passenger information requested in advance of your flight.
7a23) If you have not presented a valid ticket.
7a24) If you have not paid the fare (or any applicable surcharges, taxes, fees or charges) for your journey.
7a25) If you have presented a ticket acquired illegally.
7a26) If you have presented a ticket which you did not buy from us or our authorised agents.
7a27) If you have presented a ticket which was not issued by us or our authorised agents.
7a28) If you have presented a ticket which has been reported as being lost or stolen.
7a29) If you have presented a counterfeit ticket.
7a30) If you have presented a ticket with an alteration made neither by us nor our authorised agents.
7a31) If you have presented a spoiled, torn or damaged ticket or a ticket which has been tampered with.
7a32) If you cannot prove you are the person named in the ticket.
7a33) If you have changed your transportation without our agreement as set out in clause 3c.
7a34) If you have failed to present your ticket or your boarding pass or your travel documents to us when reasonably asked to do so.
7a35) If you have failed to complete the check-in process by the check-in deadline.
7a36) If you have failed to arrive at the boarding gate on time.
7a37) If you have refused or failed to undergo health screening or a health examination requested by us or by a government or enforcement agency.
7a38) If you have behaved in a way mentioned above on or in connection with a previous flight and we believe you may repeat this behaviour.
7b) Our right to refuse to carry you when we have banned you from our route network
7b1) We will be entitled to refuse to carry you or your baggage if we have given you a banning notice and you have bought your ticket while the ban applies.
7b2) By a banning notice we mean a written notice we have given to you informing you that you are banned from being carried on our route network. (This means you are banned from travelling on all flights we operate.) This notice will give the date when the ban comes into force and the period for which it applies. A banning notice will also ask you not to buy a ticket or ask or allow anyone to do so for you.
7b3) If you try to travel while a banning notice is in force, we will refuse to carry you and you will be entitled to an involuntary fare refund.
All of which seems quite reasonable, in my view.