Unavailable item in multiselect after one is chosen

I have to display several options in a multiselect dropdown, and some of them are related, like: year (two digit) and year (four digit). These are related options, and after one is picked, the other one must be sort of unavailable, because only one option of year displaying is possible. Question is: how to properly show that? Obviously the user should be able to still change it to the other, if they want.