You can gray out and disable an option in a select box (drop down box / list) with the following attributes.
<option disabled="true" style="color:gray;">Mercedes</option>
The option can´t be selected anymore but is still visible in the select box. This is a full code for a select box.
<select> <option>Volvo</option> <option>Saab</option> <option disabled="true" style="color:gray;">Mercedes</option> <option>Audi</option> </select>
Let’s see if this is working for you. Test the disabled option in a drop down box. Link: drop down box with a disabled option
This is very useful in a product configurator. The option is still visible instead of deleted. Normaly in a cascading drop down box the options are deleted.