There are no signs visible indicating that it's a one-way street-- you can see as far as the corner in front of you, but not to the corner behind you. A lot of people make this turn when they come out of the garage. The street is not much more than an alley, is very lightly-traveled, and has parking on one side with no-parking signs (with faces both direction) on the other. It feels like it probably should be a one-way street, but you're fairly certain it's not marked as such (and certainly isn't marked in any way that's visible to you.)
What do you do?
Edit: They were blocking the street so I couldn't go around, and I had every reason to believe that they were 100% sincere in just not wanting people to drive that way down the street. I had no sense at all that they had an ulterior motive of any sort.