My friend keeps telling me about this person I’ve repeate...

Like the other person said I'd sit em down and say "look I feel like I've been pretty clear on the fact that I don't want to be involved with this person and it feels like that boundary I've set is being consistently ignored. If it continues to happen, I will no longer respond as to maintain the boundary myself." Or whatever you feel you'll need to do to uphold it. Try to avoid using "you" statements, "you did this" "you did that", because even though it's true it can put the other person on guard to try and defend themselves instead of trying to fix the behavior. Focus on what's going on with you, and help them see it from your perspective

I suggest having a sit down conversation with your best friend and tell them how you feel very disrespected by them ignoring your boundaries. Stand your ground, but if all else fails I honestly would suggest an ultimatum