Start with the hot water fill/rinse, then fill the barrel with a solution of cold water, potassium metabisulfite, and citric acid. When you're ready to fill the barrel with beer, empty and rinse thoroughly
Per Jay Goodwin of The Rare Barrel (and Brewing Network's The Sour Hour
), they use a solution of 1 lb potassium metabisulfite and 1/2 lb citric acid per 58 gal.
If you've made three clean turns of that barrel (and the beers are still clean), then you were probably doing things right for the most part. Acetobacter is tough to get rid of, especially in a barrel, but you can limit its activity by limiting oxygen and high temperatures.
Acetobacter needs alcohol and oxygen to make acetic acid (vinegar). Limit oxygen pickup by flushing the barrel with CO2 before racking beer. Fill the barrel to the top until it overflows, then bung and clean the outside surface of the barrel. Limit sampling and popping the bung in general. You might also consider waxing a portion of the barrel to limit oxygen permeability:http://www.funkfactorygeuzeria.com/2012/02/paraffin-waxing-barrel.html
Store the barrel in a cool place, preferably between 60F and 70F. Acetobacter activity really picks up above 70F, and brettanomyces/lactic acid bacteria activity drops off below 60F.
Good luck! Please post updates!