If you still want to show the item but make it not clickable and look disabled with CSS:
CSS:
.disabled {
pointer-events:none; //This makes it not clickable
opacity:0.6; //This grays it out to look disabled
}
HTML:
<li class="disabled">Disabled List Item</li>
Also, if you are using BootStrap, they already have a class called disabled for this purpose. See this example.
As @LV98 pointed out, users could change this on the client side and submit a selection you weren’t expecting. You will want to validate at the server as well.