This trick is only intended for use with integrated ecommerce themes, not child theme layouts.

The Solution

  1. Go to Theme Options and enter the following into the Custom CSS area
    .woocommerce #left-column{width: 100%;}
    .woocommerce #right-column{display: none;}
  2. The number of columns are usually determined by the Catalog Image size you set in WooCommerce Settings on the Catalog tab. In most cases, changing it to 300 will grant 3 columns, 220 will grant 4 and so on. You will need to experiment and possibly adjust the styling to get a perfect look. If your layout is sticking to 2 columns, add the following and adjust the percentage to change the column number:
    .woocommerce ul.products li{width: 30%;}