I came here searching for a way to do just that, as there’s nothing in the Canvas settings allowing you to specify a default. I absolutely love Canvas, but most of the time I don’t want arrows in my lines and it’s tedious changing every single line.
I just saw that it’s been asked about before. It seems it’s not possible to do yet based on there not being a response, but knowing how amazing the developers are I suspect it will be added in time.