Sample Code:

<apex:page standardController="Contact" id="pg">
    <apex:form id="fm">
        <apex:inputField value="{!Contact.AccountId}" id="ifAcc" />
    </apex:form>
    <script>
    var inpAcc = document.getElementById("pg:fm:ifAcc");
    inpAcc.setAttribute("readonly","true");   
    </script>  
</apex:page>

Output: