You can renew once at immigrations (search for where and how) for a 30-day extension. If you exceed by one day, you'll still pay the fine. You have to count the days, I think - 6 months, obviously, varies in the number of days depending on the number of days in those months. I think the actual time limit is 180 days (i.e., not 6 months, not exactly a half a year at 182.5 days, except leap years)
Some people make trips to Colonia (or leave Argentina for anywhere) for a day and come back in with a new stamp in their passport on reentry. It doesn't extend your visa, but when you leave, they look at the last stamp you have as to the date you were in the country.