public static void DeleteAllItems(string site, string list)
{
    using (SPSite spSite = new SPSite(site))
    {
        using (SPWeb spWeb = spSite.OpenWeb())
        {
            StringBuilder deletebuilder = BatchCommand(spWeb.Lists[list]);
            spSite.RootWeb.ProcessBatchData(deletebuilder.ToString());
        }
    }
}

private static StringBuilder BatchCommand(SPList spList)
{
    StringBuilder deletebuilder= new StringBuilder();
    deletebuilder.Append("<?xml version=\"1.0\" encoding=\"UTF-8\"?><Batch>");
    string command = "<Method><SetList Scope=\"Request\">" + spList.ID +
        "</SetList><SetVar Name=\"ID\">{0}</SetVar><SetVar Name=\"Cmd\">Delete</SetVar></Method>";

    foreach (SPListItem item in spList.Items)
    {
        deletebuilder.Append(string.Format(command, item.ID.ToString()));
    }
    deletebuilder.Append("</Batch>");
    return deletebuilder;
}

[System.Reflection.Assembly]::Load("Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
[System.Reflection.Assembly]::Load("Microsoft.SharePoint.Portal, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
[System.Reflection.Assembly]::Load("Microsoft.SharePoint.Publishing, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
[System.Reflection.Assembly]::Load("System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")

write-host 

# Enter your configuration here
$siteUrl = "http://mysharepointsite.example.com/"
$listName = "Name of my list"
$batchSize = 1000

write-host "Opening web at $siteUrl..."

$site = new-object Microsoft.SharePoint.SPSite($siteUrl)
$web = $site.OpenWeb()
write-host "Web is: $($web.Title)"

$list = $web.Lists[$listName];
write-host "List is: $($list.Title)"

while ($list.ItemCount -gt 0)
{
  write-host "Item count: $($list.ItemCount)"

  $batch = "<?xml version=`"1.0`" encoding=`"UTF-8`"?><Batch>"
  $i = 0

  foreach ($item in $list.Items)
  {
    $i++
    write-host "`rProcessing ID: $($item.ID) ($i of $batchSize)" -nonewline

    $batch += "<Method><SetList Scope=`"Request`">$($list.ID)</SetList><SetVar Name=`"ID`">$($item.ID)</SetVar><SetVar Name=`"Cmd`">Delete</SetVar><SetVar Name=`"owsfileref`">$($item.File.ServerRelativeUrl)</SetVar></Method>"

    if ($i -ge $batchSize) { break }
  }

  $batch += "</Batch>"

  write-host

  write-host "Sending batch..."

  # We execute it 
  $result = $web.ProcessBatchData($batch)

  write-host "Emptying Recycle Bin..."

  # We remove items from recyclebin
  $web.RecycleBin.DeleteAll()

  write-host

  $list.Update()
}

write-host "Done."