Disable Lookup Input Box

<apex:page standardController="Contact" id="PageId">

<apex:form id="FormId">

<apex:inputField value="{!Contact.AccountId}" id="FieldId" />

</apex:form>

<script>

    var inp = document.getElementById("PageId:FormId:FieldId");

    inp.setAttribute("readonly","true");   

 </script>  

</apex:page>