Taxation is theft. I did not choose to buy government services, for if I did I would have a choice of which business to participate in. I do not choose to support a wasteful government charity, for if it were a charity, I would have the choice to send my money to a charity instead. It is not a voluntary collective, for I do not have the choice to not participate.
If I do not pay, I get a letter. If I do not respond, men in blue costumes come to my house to kidnap me for exercising my right to not "buy" some services or not "donate" to their charity. If I defend myself from being kidnapped, they will shoot me. I have no choice. It is theft.
Here is a more complex argument I found online:
http://www.freerepublic.com/focus/news/666806/posts