Disabling is not designed to work with type “subnm”, i.e. the whole purpose is to select an element. Not knowing your exact use case I can generally think of those options to achieve what you might want:
Limit the result to just one year, then it’s still selectable but the user cannot change it to anything else
use settingsservice true and type input, then you can disable and the selected year would come from whatever you have selected (can be a hidden fix in the background)
Use a custom html (like I described in a previous post) using a tm1-ui-element-list for instance