We have seen this problem several times, and we consider it a matter of sanitation and safety.
If there have not been any other major incidents and the only reason was that the person could not use a restroom, I would suggest a short suspension of thirty days. However, if the act was intentional, I would recommend a three-month suspension.
I would not connect the action to homelessness. Instead, I would treat it as a response to a biohazard issue that could affect the safety and well-being of staff and patrons.
I hope this helps. Let me know if you would like to talk more about it.