if you accepted payment, you must provide the service or refund the payment. basic business.
so yes, a sports franchise definitely has the right to refuse you entrance to an event... BUT... they must also refund the fee they took in exchange.
of course, if they are refusing you due to the fact they sold the seat for more money to someone else, that’s a different problem. they should not reneg on the agreement due to a better offer. that would not be smart business, as people would quickly realize they cannot be trusted to provide the service they advertised as they would sell it from under you
in the situation of a floral shop owner refusing to provide his services to a homosexual couple... that’s entirely up to him. he did not take money nor did he promise services under contract. he is not their slave and he is not obligated to provide services he does not want to provide (one of the perks of having your own company... you’re the boss. YOU decide what you want to do)
Your scenario is after the fact. For example, should the Dallas Cowboys be allowed to not sell a ticket to someone based on race or sexuality?
As to the other part of your scenario, teams already withhold enormous amounts of tickets to sell them at higher prices to agencies, with zero negative impact to their business.
On a side note, here in NY, it is illegal for you to re-sell a purchased ticket while it is legal for franchises and agencies to do the same. The game is rigged.