Why are you chasing LeBlanc? Moreover, why are you chasing anyone in Dominion? You don't chase in Dominion (well, not if you're above Bronze-level MMR).
If an enemy is fleeing, they aren't defending their node. Stop chasing and go cap the node. This forces your foe to either cease fleeing and forces a fight to defend the node. Or the enemy champion flees and you capture the node. (Or maybe another enemy arrives to defend the node...)
In any event, if an enemy is fleeing, that enemy isn't defending a node, and likely isnt attacking one of your nodes. If an enemy is fleeing to the safety of one of their points, go to another node that is undefended.
Chasing LeBlanc accomplishes nothing. If she gets away, you didn't cap the node she was defending. Or you weren't defending your node, so the enemy team is capping yours. Even if you kill LeBlanc, chances are the circumstances are the same: you're not capping -or- you're being capped.