How do I prevent the sales rep field from automatically populating to the current user? Even when I set it to ‘null’ on beforeLoad suitescript, it automatically gets set again when the Customer is selected.

Gotta run something to unset it after sourcing. It's pulling the sales rep from the entity after you clear it. You will need something client side to do it.

Use beforeSubmit to set it to null on the user event script

I got it. I had to use validatefield to set the old value to a global variable and then post sourcing to unset it if it was null during validate