disable multiple checking of checkboxes

try this code to know how to access the checkboxes in a server repeater control:
function CheckValues(source, args)
{
    //change the values of these 2 variables
    //to reflect the name of your Repeater/DataGrid
    //and CheckBox
    var ctrl = 'repContacts';
    var cbName = 'cbSelect';
   //make sure that you set this with the name
   //of your form
    var objForm = document.form1;
    var errorCount = 0;
    //get the name of the controls in the list
    var ctrlName = ctrl + '_ctl00_' + cbName;
    var chkBox = DOMCall(ctrlName);
    //check to make sure there isnt a problem
    //with your Repeater/DataGrid
    if(chkBox == null)
    {
        alert ('There was an issue checking the *'+cbName+'* check box in the *'+ctrl+'* control');
        return errorCount;
    }
    var isChecked = false;
   //very important to start at 0
    var i = 0;
   
    ctrlName = ctrl + '_ctl0' + i + '_' + cbName;
    var curItem = DOMCall(ctrlName);
    if(null == curItem)
    {
        alert ('It was null'); // something is wrong. Probably one of the 'Customize Values'
    }
    //start looping through the Repeater/DataGrid
    while (curItem != null)
    {
        //to check items inside the DataGrid/Repeater
        //using the != null check should keep it from getting locked up
        if (curItem.checked == true)
        {
            isChecked = true;                         
        } 
        //now check to see the value of i, if it's equal
        //or greater than 10 we need to change how we look
        //for the next control
        if(i >= 10)
        {
            ctrlName = ctrl + '_ctl' + i + '_' +  cbName;
        }
        else   
        {
            ctrlName = ctrl + '_ctl0' + i + '_' +  cbName;
        }     
                     
        curItem = DOMCall(ctrlName); 
        i += 1          
    }
   
    //if nothing was selected alert the user
    if (isChecked == false)
    {
        alert('Your need to select at least 1 contact in your list');
        return false;
        errorCount+=1;
    }
       return isChecked;
}
//function to use whatever DOM Object
//the users browser has
function DOMCall(name)
{
  //Checks the DOM features available
   if (document.layers) //checks document.layers
        return document.layers[name];
   else if (document.all) //checks document.all
        return document.all[name];
   else if (document.getElementById) //checks getElementById
        return document.getElementById(name);
}

//sample usage in C#
Button1.Attributes.Add("onclick","return CheckValues(); return false;");