Same here but it's not foolproof. There was a spate of robberies carried out by immigrants using the app up above in Dublin a while back. I apply the same criteria as I would to street/rank fares though i.e. if they're not fit to travel they don't travel... however, since I started working for Hail0 in 2014, I've only had cause to refuse service to two intending passengers.
I had one last night on a MyTaxi job at 5am, out comes yer-man-o with phone in hand and his mate behind him falling all over the place, he starts off with;
"Will ya take him......?" and the only thought in my mind is Rat Catchers
"Not fit to travel" quote, but I reckoned that quote was beyond comprehension to the Two lads, so I just said;
"Sorry, he is too drunk" and drove off, thankfully he cancelled me.