How can I address people that make a mess in the bathroom?

I supervise a cleaning woman at my non profit agency. We have two bathrooms that she cleans. The bathrooms are used by about 20 women. We have no men there. At least once a week someone craps all over the seats and sometimes on the floor even. The cleaning lady gets very upset when she has to clean it. We both have an idea who it may be.. we narrowed down to two ladies. I spoke to the assistant director about this about a month ago and she just told me to put signs in the bathrooms that say "please clean up after yourselves". Today it happened again. I was off and the cleaning lady texted me to tell me how pissed off she was. I had mentioned about sending a memo to everyone, but the assistant director said no. I don't think it's fair for our great cleaning lady to have to clean this up. One timeI ended up cleaning it myself. How can we address this problem?