Multiselect hides the original select and then it creates a button with the select behavior
Try
$("#myselectid").next().hide()
And it will hide that select that is created
Take a look at this page
if you do a
$("select:eq(0)")
That element is already hide but the one that you see is the next one (a button) try:
$("select:eq(0)").next()
You need to hide this one, that is the one created by multiselect