Certain players cannot be de-selected for special teams
 

No idea if this is patch-connected -- have never seen this until today. (Patch 8.1 came out a few days ago, I think this may be my first time doing this since then)

I'm going through my roster assigning ST duty, and I have two players who refuse to switch from Y to N. Neither one is injured, nor anything else I can see that seems like it should affect this. The rest of the roster responded fine to my direction.

Familiar? Any clues?

This is was an issue pre patch. Normally after another sim you will be able to switch them as per normal.

How does one assign/un-assign a player to special teams duty?

On the player attributes and special teams screen, which you get to from the roster screen. Right click on players to turn their 'special teams' duties from Y(es) to N(o) or vice versa.