Simple, it would treat those jobs as under minus sign(i.e. not available) and pops on them would be invisible to job selection algo.how well the system could adapt to "fixed pops" that the game can't automate until the player removes said "fixed" tag
But showing in interface who's fixed and who's not, especially in multispecies empire, would be a nightmare of UI clutter. As if there's not enough of that already...