Get yourself one of those huge big lamps that turn on when they detect movement. We used to have people cutting through our garden all the time and they don’t like it when they are lit up by a million candle spotlight.
It also means you can see them and they can’t see you.
Also I would phone environmental helath to get them to clean up. Its far more obvious than you cleaning them up yourself and maybe this wil make them think of somewhere else to go, as if it is someone from that house then they are obviously trying to hide it from the rest of their family.