Then the Trump-Kennedy center will continue to have “last minute” cancellations - obviously done on purpose to disrupt the schedule and “take a stand against Trump.”
Have them sign contracts for performances so they can then be sued for breech of contract.
They are already sign contracts and they have been willing to break them to virtue signal and are doing it for the reasons you gave—to hurt Trump and MAGA and to keep their membership in the Liberal Club. Part of it in their eyes is survival long-term. If they were to perform at the Trump Center they might not get jobs at other venues, might not get recording contracts, might tick off investors, etc. It’s a calculated risk that they feel will benefit them long term.