An asp:Panel just produces a div element. This isn't a form control, it's just there for structure.
To disable every input control inside of it, if you are using jQuery, try:
$("#<%=mypanel.ClientID%> input").attr("disabled", true);
var controls = document.getElementById("<%=mypanel.ClientID%>").getElementsByTagName("input");
for (var i = 0; i < controls.length; i++)
controls[i].disabled = true;
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