How to change the Zen theme to a single sidebar layout?
In page.tpl.php, at line 109 and line 110, delete the sidebar you don't want

In style.css change this class
.region-sidebar-second{
float:left;width:20%;margin-left:0%;margin-right:-20%
} 

to be
.region-sidebar-second{
float:left;width:20%;margin-left:0%;margin-right:-20%;display:none;//add a display:none
} 

and change .two-sidebars #content {
float: left;
width: 60%;
margin-left: 20%;
margin-right: -80%;
}

to be

.two-sidebars #content {
float: left;
width: 80%; //change 60% to 80%
margin-left: 20%;
margin-right: -80%;
}