sold my old I phone to a scammer how do I lock it so they can't use it

Assuming you followed the procedure for selling an iPhone and restored to to factory settings, there is nothing you can do to lock it.

I've seen recommendations to call your carrier and have the device blacklisted for other situations, I'm not sure what the requirements are to have this done, but you could call your carrier and inquire about doing that.

Also consider filing a report with your local police department.

what makes you think this person is a scammer?

that's what I'm not entirely sure about.. if they only blacklist the device when they still have ownership of it or what.

Maybe someone else will see the thread and comment.