Would you let someone leave a bad tip?

I live in Australia where it's very rare to give/receive tips in restaurants, let alone anywhere else. The way I see it, any amount of extra money from the customers should be kindly accepted. Really, it's sort of ridiculous for waiters/waitresses to now EXPECT a tip when the whole reason it started was as a kind gesture. Now that tipping is such a common custom in a number of countries, it would be rude not to give one, but I just wish that people would take a step back and realise that tips ARE voluntary. Kind gestures, big or small, should never end with the receiver being offended or dissatisfied. THAT is rude.