Modal Popup extender keep displaying after validation

I have a modal pupup extender that calls javascript on a button click event within the MPE.  The javascript validates to make sure that a field is valued and displays an alert if the value is not there.  After clicking the alert, the MPE closes.  I would like to keep the MPE open, so the user can add the info.  Can this be done:

javascript:

function ValidateAdmin(){

var ddl = document.getElementById("<%=ddlAdmin.ClientID%>");

var SelValue = ddl.options[ddl.selectedIndex}.text;

if(!SelVal){

alert("Please select a person to add.");

return false;

}

}