Never one to allow a mistake to go uncompounded by a glaring error, Bob Murphy digs in deeper. He claims that "Taking money from people against their will is not akin to getting on the treadmill; it is akin to killing people against their will."
Bob has introduced a largely irrelevant criterion here with his "against their will." Let us start with killing. (No, no, not killing Bob: we still love him despite his obstinacy.)
The justice of a killing does not depend at all on whether the "victim" wants to be killed. If I shoot someone who is attempting to set off a nuclear weapon in Times Square, the fact that I killed him "against his will" does not make my killing immoral. And if a friend who is in despair asks me to shoot him in the head, the fact that he wants me to kill him would not make my action moral.
Similarly, in taking money from people, the crucial question is whether you are taking it justly or unjustly, not whether they want you to take it or not. So, for instance, the people on late-night TV who sell silver dollars worth $10 to unsuspecting dupes for $30 are acting immorally, even though the dupes are sending them their money willingly. And if my negligence causes you to break your leg and you've won a lawsuit against me as the result, it is just for you to take the settlement from me, even if I don't willingly pay up.
In fact, we can see that in anarcho-capitalism, money would be taken from people "against their will" all the time, in lawsuits, alimony settlements, child-support payments, and so forth. In fact, anarcho-communists living in ancapistan would probably find every single payment they make to be "against their will," given that they are being forced to pay people money due to the institution of private property, which they find to be immoral.
So the whole "against their will" point is a red herring: the real question is, "Is the state acting justly or unjustly in collecting taxes?" (Note that one might find the concept of taxation to be fine, but still find some levels of taxation to be unjustly high, or hold that taxation is fine to pay for some government services, but not others.)
Of course, that is a trickier question to deal with, which is really the whole point of the "taking money from people at gunpoint" rhetoric: to turn a tricky question into a simplistic slogan!