The basic system under the Republic was that groups of publicani would bid for the right to collect taxes in a given province. Whoever offered the Senate the highest bid would pay that amount to the Senate up-front, and then spend the rest of the year trying to recoup all of that money and then make a profit on top of that. They were very good at it, and the Senate didn't really care to get in the way. Keep in mind that Italy didn't pay taxes after the Punic Wars, so this was really just the provincials' problem. Governors were also known to try to bleed their provinces dry.

Part of the problem is that any restrictions or regulations were enforced through the Roman courts. Citizens brought other citizens to court, so the provincials would have to sent some people to Rome in the hopes that they could find a citizen willing to spend time and money to try to convince a Roman jury to favor barbarian peasants over a rich and well-connected Roman. It wasn't impossible - Cicero represented Sicilians against their governor - but it was a system that favored taxes over subjects.

To give you an idea of how hated this system and the publicani were, go read up on the Asian Vespers. When Mithradates Eupator, the king of Pontus, invaded Roman territory in what's now Greece and Turkey, the provincials not only surrendered without a fight, they massacred every Roman they could get their hands on. Supposedly, Mithradates tried to give a Roman ambassador his fill of gold - 'even though the Romans are the greediest of sll people' - by having molten gold poured down his throat.

I suspect that Augustus reformed this system a little before whatever period you were talking about in Sunday school, but someone else will have to fill in the details.