I read that Agenda effects and abilities cannot be prevented or cancelled.

With the Heir to the Iron Throne Agenda you may initiate an extra power challenge per turn. Can this be prevented with card effects that limit the amount of challenges you can initiaite?

For example, Conventions of War Plot (5 Kings) is in play (After a player has initiated 2 challenges they may not initiate any further challenges that phase)

I have the HTTIT Agenda and initiate a Military challenge then a Power challenge. Can I now initaite an additional power challenge as per my agenda or does the plot prevent me from doing this?

Marshal Lambert said:

So, if there was a card effect that reduced the draw cap from 3 to 0 you wouldn't be able to draw an extra card with the Knights of the Realm agenda?

Correct. Same deal: imposing a restriction that doesn't let the effect initiate in the first place is not the same thing as waiting for the effect to initiate, then interrupting it before it can resolve (which is the definition of "cancel").

Marshal Lambert said:

That's a misconception right there. The FAQ tells us that effects of an Agenda cannot be cancelled. It says nothing about "prevented" (which is not a game term anyway).

For example:

You're running Siege of Winterfell. Your opponent can't use cards like To Be A Kraken or Seasick or He Calls it Thinking to cancel the Response on the card. So far, so good. But if you win a MIL challenge and PotS Brienne of Tarth participated on your opponent's side in that challenge, you can't trigger the response on SoW, because Brienne prevents you from doing so. Brienne's effect is not a cancel, so she works on SoW. See the difference?