Is this a sneaky trick question perchance? Being that through much of the Classical period citizens of Rome were exempt from the majority of taxation, especially those living within the city itself..

Assuming you mean more generally across the Empire/Republic. The answer is it depends which tax you are talking about and who you were. The Jews for example had punitive taxes placed on them at various points and clearly non-payment of these might be more serious than say not paying a more benign tax as a Roman citizen (to the extent they paid at all).

In many provinces where there were taxes being imposed on non-citizens taxation was farmed out and if you didn't pay the tax farmer's rather threatening men would probably come and have a quiet word... Prime example is Judea around 1 CE, the Taxes were collected by locals not by the Romans directly, they were popularly detested and vilified for collaborating with the foreign power, reputed unfair taxation and self-enrichment etc. This type of individual almost certainly (for his own protection as much as anything) probably would've had a suitable number of toughs to assure people paid up on time.