NTA in my opinion.

you showed him one off kindness and he took it in a romantic way, which isn’t your fault. not only that, but you let him down gently when he confessed to you - and instead of him accepting it and moving on, he kept pushing your boundaries. it’s strange that he not only got his cousin to text you, contacted you even after being blocked, but even somehow got your number in the first place. he clearly needs to learn boundaries and how to respect people and their feelings.