Remember if you will for a moment my post that mentions how events are self fulfilling and every event has a ripple in all directions in time to make sure it happens. This is because in a strange thought the future events actually can change past events under the right conditions, though they would have to be more substantial as time is a force in motion and thus force moving backward is weaker than force forward. Similar to pushing vs pulling or multiplied acceleration, in which if you normally throw say a baseball at 40 ish mph, while you may throw it at the same speed from a moving car by technicality it still is moving faster than normal if you throw it in the same direction you're moving or slower if you throw it away in opposition.
So to stop an event from happening you have to either make last minute type alterations before its ripples have a change to change things in which you'd have to know an event is going to happen for the first time before it happens, or you'd have to make stronger events happen around it that would throw it out of balance or disrupt it until it can't be sustained. Small alterations and even some more serious direct intervention if it doesn't completely destabilize an event will be ignored as the next time the event occurs the ripples will make alterations to stabilize itself and ignore your changes. So think about an event if you will as with a color. To destabilize an event you have to first remove its supports by altering complimentary color events thus isolating it. You must then find the furthest point of reach in which it starts to make itself self fulfilling and alter it, weaken it, eliminate it if possible. You then track subsequent similar events and eliminate them and if possible make opposing color type events or substantially stronger same color type events to negate it.
Let's say you wanted to prevent an accident. You could create a new event causing a different accident, or perhaps create an obstacle causing a dilation or distortion altering the timing which can destroy the event if none of the parts are in place anymore, or any number of other things. The furthest point of this say car accident to be more specific would likely be the responsible vehicle, and thus by removing it by say deflating the tire a bit or removing gas or delaying the driver at any point before he gets in the car just prior to the accident would likely suffice for this weak force time event. The principals for a large scale strong force event are similar. If you wanted to stop WW2 trust me killing Hitler wouldn't be enough, I tried it once. In fact the ripples of WW2 and WW1 are tied strongly together and you'd actually have to prevent both to prevent either. The easiest solution to that particular problem actually would be to relieve the pressure as a metaphor by forcing a different war to happen prior. Though obviously that's not the only way and admittedly kind of defeats the purpose.
So simple recap is isolate and attack really. Assuming that the event lacks a single focal point vulnerable to direct intervention.