Can a photographer sue me for not signing a contract after I decided to end all future collaboration with him?

There are too many details missing in your question for me to give you a good answer. On the face of your question, it seems you are concerned that you were required to have a contract with the photographer. The law does not require people to have written contracts, generally, but having a written contract is a good idea. It helps to establish the expectations and may provide a framework for resolving differences if written well. Misunderstandings often arise when two people think they understand each other and come to find out that they have two very different sets of expectations. Putting an agreement in writing can eliminate those misunderstandings by requiring you to work through things ahead of time. It sounds like the photographer expected you to stay with him/her for a longer period of time. Having a contract that states when the agreement terminates and how it can be terminated would have been helpful. As the first answer states, our laws guaranty people access to the court system when they think they have been wronged. You cannot stop someone from suing you. That does not mean they have a good case. If it is as cut and dry as you think it is, the case should not go very far. If there is more to the story, and in my experience there usually is, then who is right and who is wrong will be decided by a judge or jury. This answer is not intended to create an attorney/client relationship and is not intended as legal advice for the specific situation. I have just stated general principals. The unique facts of your situation should be presented to an attorney who can give you specific legal advice.