https://youtu.be/LDfPvOKk2WI

You can watch the video for what happened, but I'll explain.

I'm lv. 7 Cleric, playing OC.

There's a place called 'Warrens of the Damned' at Beggar's Nest (https://www.gamebanshee.com/neverwinternights/nwnwalkthrough/warrensofthedamned.php)

In this place, there are 'some of the zombies that you will find here and give will try to warn you to flee this place'. These zombies are considered NPCs, not enemies. It won't attack you, you can't attack it.

However, I've used Turn Undead and it turned into hostile. But my attacks are not effective so basically this zombie is invincible.

How do I kill it, or get rid of it?

As far as I know, you can't. You are not supposed to attack it, making it consequently hostile.

However you might be able to lock it in a room or block it somehow with a spell. Maybe, I repeat, maybe, if you go away and come back later, the zombie NPC could turn back to friendly.
Otherwise, I suggest you to load a previous save and avoid attacking it.

I didn't intend to attack it. There were other zombies nearby I wanted to attack them only.

I just loaded my last save file. Thank you.