Governments have a legal process for collecting taxes. It isn’t criminal and therefore is not theft.
The counter to this may be that people reject the authority of the state and any moral basis for collecting taxes.
However it is the authority of the state that also defines what is legal. If government are not a valid authority then there is no legal basis for decrying anything as theft as there is no authority to declare anything as legal or illegal.
Either way the real argument of “Taxation = theft” really amounts to “I don’t like taxes”, to which the rebuttal is “Yeah, I don’t care what you like or dislike”.