Does it say in your contract that you may terminate if the relationship because abusive/disrespectful to you or the dck's and/or due to continous late payments? If so I would keep the deposit as they have violated the terms of your contract. You will need to fill this space and they have left you with really no choice but to terminate. They have cost you income. It is not unprofessional to keep what is rightfully yours. Good luck and I good for you for getting rid of these people.