People complain about cats defecating in their plants and this is certainly a problem which must be solved. Just today I have had two complaints about cats defecating in plants. While we will definitely try our best to help, and we do sympathise, it's important to remember that the cats are defecating there, because someone is (i) letting their home cats roam or (ii) feeding the cats upstairs. Removing the cats will not help if this continues since new cats will just move in due to the vacuum effect.
What annoys me is when people try to throw the book at the feeders or flat owners and say that cats are not allowed in flats. Clearly these people need to be educated. However, you know what they say about people in glass houses not throwing stones? Well, neither are plants if they obstruct the corridor. A wider reading would suggest placing anything in a common area is not allowed. Check your town council by-laws.