I even threatened him that if he continued to do so, I would considered that action as he wanted to make up and be in a relationship with me again. But he didn't care and kept adding friends me.
So I added him once and when I started the conversation, asking why he does so, he ignored me. So I unfriended him and then the cycle started again: he added me back.
This time I blocked him for good. So annoying. I must admit that I still have feelings for him. But why does he keep messing with me like that?