It's already illegal to pay federal taxes:
22 U.S. Code § 2780 - Transactions with countries supporting acts of international terrorism
In particular:
(b) Prohibited transactions by United States persons
(1) In general
A United States person may not take any of the following actions:
(D) Taking any other action which would facilitate the acquisition, directly or indirectly, of any munitions item by the government of any country described in subsection (d) of this section, or any person acting on behalf of that government, if the United States person has reason to know that that action will facilitate the acquisition of that item by such a government or person.
Why would a US person have reason to believe that the above is being violated by the US?
History: Iran/Contra, Syria, Libya, ISIS, etc.
I’m afraid that would only be true if the legal system followed actual logic. As it is, the incoherent mess of “precedents” means largely that the government (and the judiciary) can decide whatever it likes, without regard for logic or for a public mandate. (Cf. Obamacare “tax”)