In a slide layer, how to I remove the next button?
In Storyline 1, you can only hide the built-in nav buttons for a specific slide (not for a layer). You could 'disable' the Next button when a layer is being displayed, by using a variable is set to True whenever a layer is active and set to False if the baselayer is active. Then attach a condition to the Next button so that it only jumps to the next slide if the variable is false.
In Storyline 2, you can use triggers on a layer to change the state of the Prev or Next button to either Hide or Disable.
As Michael stated, you cannot hide the navigation buttons from a layer level.
Here is some information on how to disable the navigation temporarily.