I'd just politely explain that you really prefer she didn't stab you with things. You don't have to lecture her or explain that it's dangerous, just tell her that it probably hurts more than she thought it would.

I have a good female friend who also does things like this, but I like it, so eh. She knows when to stop and never hurts me too much, so it's all good.

I'd just politely explain that you really prefer she didn't stab you with things. 

"Hi, yes, please don't stab me with things. Thanks. "

There was once a girl i knew. She wasnt a very nice person.

She stabbed me in my arm repeated with a sharp piece of wood.

She also bit my arm.

But I didnt really do anything about it, so i suppose it is my fault.

I still have the scars.