Isoamyl acetate is the ester responsible for the banana-flavor.  Typically described as the “circus peanuts” flavor or the dominate flavor in german-style hefeweizens.

How does it form in beer?

This compound forms by the condensation of acetyl CoA and isoamyl alcohol during fermentation.  Most esters form in beer due to the reduction of a carboxylic acid and ethanol.  This reaction occurs at higher temperatures and is therefore most common in top-fermenting yeast.  This is the reason why ales tend to have more estery and fruity flavors than lagers.

How do you solve the problem?

Some yeast naturally produces isoamyl acetate (i.e. german hefeweizen yeast), but most yeast will produce a variety of esters when fermentation occurs at a higher temperature.  It’s important to control your fermentation temperature or use cultured yeast that doesn’t give off as many ester precursors.