I would say just tell them to f off in the nicest way possible, even though they may not be doing it to offend. I worked with a Chilean woman in the States and a relative of her husband would always makes fun of/mock her accent when she spoke English. She never liked it and finally got fed up enough to say something. The relative said she felt really bad and that she meant no offense. In fact, she thought my Chilean's co-worker's accent was charming and that's why she imitated it. So you never quite know why folks do that kind of thing Having said all that, if it bothers you, say something.
I have one friend here who always imitates my accent, even though I have said something to him. I am really surprised that I don't get that more, not because Argentinians are cruel people, but because human beings in general often look for and exploit weakness or difference. I find most folks here to be very tolerate of foreign accents, including mine.