How to power off Samsung A6 without screen
You can take out the battery, as shown here:

https://www.youtube.com/watch?v=oVl9EGJdTHE

if you are willing to DIY 

(You might need to go that route anyway, if you plan to replace the screen yourself -- which is probably much cheaper and faster than using a repair shop)

P.S. Actually, it's probably best to let the battery drain before opening the phone, but I'm not sure.