I have a level 10 Orc Fighter. Among other things, I have the Combat Patrol feat.
As a full-round action, you may set up a combat patrol, increasing your threatened area by 5 feet for every 5 points of your base attack bonus. Until the beginning of your next turn, you may make attacks of opportunity against any opponent in this threatened area that provokes attacks of opportunity. You may move as part of these attacks, provided your total movement before your next turn does not exceed your speed. Any movement you make provokes attacks of opportunity as normal.
I set up my Combat Patrol, in front of our bard. An enemy attempts to charge that bard.
You must have a clear path toward the opponent, and nothing can hinder your movement (such as difficult terrain or obstacles). You must move to the closest space from which you can attack the opponent. If this space is occupied or otherwise blocked, you can’t charge. If any line from your starting space to the ending space passes through a square that blocks movement, slows movement, or contains a creature (even an ally), you can’t charge. Helpless creatures don’t stop a charge.
As part of the attack of opportunity I get, I'm allowed to move due to Combat Patrol. I stand in front of the charging enemy as part of my Attack of Opportunity. Does this stop/interrupt the charge?
On one hand, the action is already on-going, and even if it's just my limited knowledge, I don't know any other case where an on-going action is invalidated. (Even counterspells just negate the spell, but don't interrupt the action of spell-casting.)
On the other hand, that path is still blocked by 234 lbs of Orc Fighter, and at the very least, that would have prevented the enemy from starting that action at this moment.