A tax on aggression — like a pollution tax, or other Pigovian taxes on negative externalities — is not itself aggression.
A tax on the economic rent of land is aggression only if you disagree with the geolibertarian premise that because land is not created by labor, the occupation of land does not include an entitlement to its economic rent. (The economic rent of a piece of land is equal to the economic advantage obtained by using the site in its most productive use, relative to the advantage obtained by using marginal land for the same purpose, given the same inputs of labor and capital.) Land rent is just one of many questions about property rights that cannot be settled by invoking the non-aggression axiom. Other questions relate to atmosphere, bodies and streams of water, rain, sunlight, wind, migratory game, fisheries, minerals, spectrum, orbits, expressions, inventions, and reputations. Depending on your axioms about the nature of property, any or all of these could in principle be taxed without committing aggression.
Thus one could assert "land rent is theft" and say that morality requires an exaction (i.e. tax) against land rent. One could similarly assert "pollution is theft" in defense of taxing negative externalities. An anti-aggressionist could even assert "free-riding is theft" in defense of a tax levied only to fund a "public good" (i.e. a non-rivalrous non-excludable product or service) for minimizing aggression — such as national defense or guaranteed access to the justice system. As geolibertarian economist Fred Foldvary says: "government works and services increase land value, and so long as these are provided and funded by government, a levy based on the site value returns to government that land value and rent added by the services." A geolibertarian could argue that a tax on land rent is not strictly coercive, but rather a form of restitution.
Taxes used to finance the protection of liberty are fundamentally different from taxes used to finance rent-seeking (i.e. extracting undeserved benefits from the government). If taxation to finance justice for all is so horrible, so akin to "theft" and "slavery", then any form of taxation should be able to serve as the poster child for the anarcholibertarian argument against coercive minarchism. If instead anarcholibertarians have to paint an image of John Q. Taxpayer being forced at gunpoint to redistribute his income to others, then they just aren't engaging my position. The image I defend is of John Q. being forced at gunpoint to pay his pollution taxes (corresponding roughly to his pollution aggression) and his land value taxes (corresponding roughly to the services available for him to free-ride on). Can anarcholibertarians argue against that, or not?
Saying taxation is a form of confiscation is like saying assault is a form of murder, in that only 100% taxation qualifies as actually confiscating the object of the tax. Just because taxation and confiscation both involve threatened force doesn't make them the same thing.
Seize/confiscate has connotations that are not satisfied by taxation. One is that the seized/confiscated thing is a distinct entity or collection that was the particular target of an intent to take possession of it. A mafia goon might "seize" or "confiscate" your wallet or all the cash you happen to have in your pockets, but if instead he says he'll be back each week for $100 in protection money, a native speaker of English would instead say he is "exacting" or "extorting" when he collects it, not "seizing" or "confiscating". The object of a seizure or confiscation is a particular thing that you in some way possess at the time of the threat or exercise of the force involved in the seizure or confiscation. By contrast, taxation/exaction/extortion often imposes a debt that is satisfiable not by any particular thing(s) you currently possess, but rather by some dollar-denominated (but otherwise arbitrary) subset of your future earnings or holdings. The goon doesn't care which hundred-dollar bill you give him next week, or that you don't have any now, so long as you cough one up each week. Thus taxation is in form more like extortion than it is like theft.
Most Libertarians seem to recognize that coercive taxation — or something that could be denounced as such — will be necessary indefinitely. Of the the nine LP presidential tickets, at least seven were headed by men who conceded (then or later) that coercive taxation will be necessary indefinitely — rejecting the pre-Portland Platform's call for abolition of all taxation and immediate non-enforcement of tax laws. Andre Marrou may merely have opposed "excessive taxation", which would make it 8 out of 9. And while David Bergland was a Rothbardian radical when nominated in 1984, by 2000 he was managing the campaign of Harry Browne, who wrote at the time that "until we find a way to finance government without taxes or a way to assure our safety without any government, some form of taxation will be necessary". So it might actually be 9 out of 9.