Someone is throwing dog poo into my mum's plants...what should we do?

We live in a flat and my mum has loads of plants in the corridor cause she loves gardening, plus we use them for cooking. Lately though, some weirdo has been throwing their dog's poo into the pot of a particular plant, and we can't think of who it could be! we tried putting up a sign and it worked cause it stopped for a while, but there was poo again this morning. My mum's getting really frustrated and upset...any ideas on what we could do?