I don't usually contend with libertarians. The first reason is that I agree with them on perhaps 80% of the issues maybe more. The second reason is that they are a marginal and somewhat irrelevant faction in both American and world politics. Most of their criticisms of government are robust, accurate and need to be brought to the attention of the public, but a few others verge on the inane by oversimplifying ethical quandaries that aren't black and white in every instance. Taxation is one of them. But before we can tackle this issue we have to have a consensus on what constitutes property. Your property, in everyday life, is the product of your labor. There are exceptions such as parents bequeathing their property to their children or people receiving gifts on special occasions. When a person transfers the product of their labor to another without monetary gain we must still acknowledge the recipient as the rightful owner to respect the will of the benefactor, but in everyday life your property is what you earn. However, you do not own your money. What you call your money is simply promissory notes backed by the faith that they will continue to be accepted in exchange for tangible goods and services. When you make a bank deposit you don't own the amount on your bank statement, you only have a claim to demand that much money if you wish to withdraw. The relationship between a depositor and a bank is that between a creditor and a debtor. If the money were actually your property it would be more of a beneficiary/trustee relationship, but the bank can invest and lend this money in whatever manner it sees fit without your consent, so its not your property in any real sense of the term. Furthermore, your promissory notes wouldn't have any value without taxation. It is by forcing individuals to invest in public services with these promissory notes that makes them desirable to businesses in the first place. Otherwise its just scratch paper. Actual property cannot be used or transferred without the owners consent; It has value even if it isn't taxed, and its more than a claim to be redeemed in the future. This is not an endorsement of fractional reserve banking, only an explanation of why simple tautologies cannot capture the whole truth of the matter. Taxation is more akin to economic coercion than theft, since an individual is subject to deprivation if he doesn't have any dollars.
There are some taxes that aren't simply the arbitrary taking of property but function as public user fees and are paid voluntarily. Land value taxes return rent created by commerce, infrastructure projects, public services, and population growth back to the community that created it, and thus functions as a user fee for having a monopoly over certain land. Taxing the royalty income of patentees would follow the same principle. A severance tax functions in the same manner as a land value tax; it is a user fee of a nation's natural resources. The excise tax you pay at the pump is a sort of user fee for the interstate highway system, and like the aforementioned taxes its paid voluntarily. If you don't want to pay it, you can choose some other mode of transportation. The same could be said of fishing and hunting licenses, which are used to restock lakes and preserve species for hunters in the future. Even in an anarchistic society you would still have to pay for monopolistic services. For instance, homeowners in a gated subdivision don't each have their own security company. It is far cheaper for them to share one security company than for each of them to hire a separate security company. The same would be true of services such as sewage treatment, roads, and water.
Saying taxation is theft is akin to saying killing is always murder. It's not necessarily true. Libertarians that spout this as a tautology remind me of the SJWs that say all white people are racists or the feminists that say all drunk intercourse is rape. It's just another moral absolutist ideology that's incapable of accounting the for intricacies and variations of life.