How can I disable alerts?

If you are using Update Panel then add the following lines after the script manager:

Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequest);

function endRequest(sender, args)
{
var e = args.get_error();

if (e != null)
{
args.set_errorHandled(true);
}
}

If Web Service, provide an onFailureCallback.

Sorry my ignorance, but I have to open javascripts Tags?

<script>
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequest);
function endRequest(sender, args)
{
var e = args.get_error();

if (e != null)
{
args.set_errorHandled(true);
}
}
</script>

Yes you have to add it in an script tag after the script manager.

Ok. Now it shows the mensage: Error: 'Sys.WebForms.PageRequestManager' is null or not an object.
Do you know whats how can I solve it?

Did you add it after the ScriptManager? Would you mind posting the markup of your page

I found some about hidden alerts. I can put a error callback on function like this:

functionErrorCallBack()

{

}

function DoItFunction ()

{

AjaxServicesExemple.DoItMethod(onDoItMethod,ErrorCallBack);

}

functiononDoItMehod (result)

{

var using = result;

}

Thanks a lot guys...