@Leah39 As a host, I think you should always go with your gut feeling, if you don't you'll just be anxious and may find the place a mess or damaged after they leave.
Everyone has different comfort levels with the drug issue. Myself, and plenty of other hosts have no issue with people smoking marijuana, altho most don't want smoking inside the unit, for the lingering smell factor. Someone doing heavy drugs, or even drinking to excess, is a different story.
But a guest who immediately hangs a blanket over the door, doesn't come to the door and greet you in a friendly way, and seems to be trying to hide what they're doing, doesn't sound like good news, aside from whatever they've got in the bong.