mtg, mtg cards, magic the gathering, gaming, geeks
Never mind, Gatherer has a ruling:
4/15/2013 If a source an opponent controls would deal noncombat damage to you, you may choose to have Riot Control prevent that damage before that opponent may choose to redirect that damage to a planeswalker you control.
So, they redirect the damage as it would be dealt? In other words if the damage was already prevented by a "shield" like Riot Control , there won't be any left to redirect?
MinscAndBoo, the replacement effect of preventing the damage happens before the replacement effect of redirecting the damage, so there is no damage left to redirect. In other words, Riot Control would prevent the damage from happening before it could be redirected.
However, it would not stop combat damage from creatures attacking Ajani, because that damage goes directly to him and is never affected by Riot Control .
To add to what raithe000 said, the replacement effect of preventing the damage to you doesn't necessarily happen before the redirection of damage occurs - you (as the affected player) get to choose which replacement effects to apply in which order. Of cause, usually will want to prevent the damage.
Example1:If you choose "Prevent all damage from you" as first redirection effect, the second one ("Your opponent may choose to redirect all damage to a planeswalker you control") is no longer applicable, since there is no longer damage to redirect.
Example2:If you choose (for whatever reason) to use "Your opponent may choose to redirect all damage to a planeswalker you control" as your first replacement effect AND your opponent chooses to deal the damage to your planeswalker, the "Prevent all damage from you" effect is no longer applicable and the planeswalker will receive the damage.