This can be done by exposing a spacing property for the particular Series. 

For the time being mentioned properties can be set on the client through the chartObject of the RadHtmlChart:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script type="text/javascript">
			function ChangeSpace() {
				var chart = $find("<%=ColumnChart.ClientID%>");
				chart._chartObject.options.series[0].spacing = 0.9;
				chart.repaint();
			}

			function ChangeGap() {
				var chart = $find("<%=ColumnChart.ClientID%>");
				chart._chartObject.options.series[0].gap = 10;
				chart.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
		<telerik:RadButton ID="RadButton1" runat="server" AutoPostBack="false" Text="Change Space" OnClientClicked="ChangeSpace" />
		<telerik:RadButton ID="RadButton2" runat="server" AutoPostBack="false" Text="Change Gap" OnClientClicked="ChangeGap" />
		<telerik:RadHtmlChart runat="server" ID="ColumnChart" Transitions="true">
....
		</telerik:RadHtmlChart>