When I was out on my motorbike many years back, I had someone empty a kiddies potty out the back window.
Luckily (?) it only contained liquids and no solids / semi-solids and most of it missed me. Even so, I was obviously not very chuffed and expresed my disapproval to the occupants as I came alongside them and I was greeted with a 2 finger salute from the (supposed) guilty party. I did think about trying to force them to stop somehow and "have words", but the driver looked to be a bit of a greb, so I thought better of it and passed them and left. I did think about reporting it, but decided that it would probaly be a waste of time and I didn't bother (my word against theirs and all that).