Tomas,
You have CSS within your themes style.css file that is adding a 20px margin to the bottom of the columns. You would have to overwrite this in the css that targets screen widths
media="all"
.five-sixths, .four-fifths, .four-sixths, .one-fifth, .one-fourth, .one-half, .one-sixth, .one-third, .three-fifths, .three-fourths, .three-sixths, .two-fifths, .two-fourths, .two-sixths, .two-thirds {
float: left;
margin: 0 0 20px;
padding-left: 3%;
@media only screen and (max-width: 600px)
.five-sixths, .four-fifths, .four-sixths, .one-fifth, .one-fourth, .one-half, .one-sixth, .one-third, .three-fifths, .three-fourths, .three-sixths, .two-fifths, .two-fourths, .two-sixths, .two-thirds {
padding: 0;
width: 100%;
}
Try adding margin-bottom: 0px !important;
to the last statement above.