Well, really there is no way to debate with a statist because they are braindead to begin with, but here are some good talking points.
Taxation is coercive, since if you do not pay your taxes, you are kidnapped at gunpoint and thrown in jail – where if you try to escape, you are shot.
Being offered a choice between two violent alternatives is not the same as being free to choose. If a store owner gets to choose which Mafia gang he pays “protection” money to, can it be really argued that he is making a “free” choice? If a woman can choose between two potential husbands – but will be forced to marry one of them – can she said to be really “choosing” marriage? People can only freely choose governments, if they have the choice not to choose governments.