Q: Can someone play an AO Special (Negates the effects of any one special card) to Negate the effects of a Special card being played because of an Activator? I'm not sure that I'm phrasing this question properly, but you get the idea.

A: How to play negates 101:

If it is your turn, you can play an AO Special to negate any Special card that your opponent played that is still in effect (like a hit on your record, or a card that allows one of his characters to have a special ability).

If it is not your turn, then you may play a negate in reaction to a Special card that your opponent plays, even if it doesn't affect you (like a Draw 3 cards).

You may not use on your turn to negate a defensive action played by your opponent. So you cannot negate an "avoid 1 attack" special.

Nothing distinguidhes Special cards played by activator as opposed to other specials. 

A: Oh sure, there's always an opportunity to defend an attack before it hits, even if it wouldn't be defendable that way after it hits.  Here's the metarule that backs me up on this:

(110) A Special that indicates what can and cannot be played once it hits can still be defended by the type of card it restricts, provided no other restrictions are in play.