How does trample interact with damage prevention effects and indestructible blockers?
Damage assignment ignores damage prevention and damage-modifying effects. Just look at toughness, previous damage marked, and deathtouch to figure out how to allocate trample damage (or damage in a group block).

Here's the most relevant bit of comp rules:


  510.1c ... When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures
  that’s being assigned during the same combat damage step, but not any
  abilities or effects that might change the amount of damage that’s
  actually dealt.


In other words, you only have to assign damage that would be lethal -- based on the creature's toughness adjusted for any damage already marked on it, or 1 point of deathtouch damage -- without adjusting for other game rules like damage prevention, damage multipliers, or indestructibility. So my 4/4 can trample right over your Fog Bank as if it were any other 0/2; the Wall just won't die in the process.

As a side note, since there seems to be a bit of confusion about this in the question: protection is a damage prevention effect. So don't go blocking any Excruciators with your Kor Firewalker if you want him to live.