I Can't stand it anymore.

[Slartibartfast]

There you go mate. Roleplay the situation and if you fight and the other guy just keeps running, you can break off and let him run. If he decides to stay within range to loophole the rules you simply do the most easiest thing in the world.
Bug out and let him be. Roleplay the situation and forget it. Screenshot the event and the RP and mend your way. Roleplay with others instead of being afraid of "oh if I move now I have technically fled!"

If the other guy just waits so he can delcare you fleeing, then he is in conflict with 0.0
Sanctions are not a goal to achieve and are not here to be used for your own amusement or advantage.
0.0 can hit people verry hard for this kind of loopholing stuff.

Roleplay the game instead of wasting time and get frustrated by such incidents.

Yeah, I had the very same situation just yesterday...

The guy runs off, but stays in system. I very politely ask him shouldn't he be out of here, but he says I docked before he went 10k away from me... And i honestly didn't know if that was true or not. So what to do in these situations? In the end we both agreed just to ignore the whole conflict, but what if we wouldn't?

I mean, I can't screenshot every second of my ingame time just in case someone would try to loophole me or something...