how to say no

I do not mind adding strangers as my friends on Facebook as long as these strangers are tango dancers who organize milongas.
I do mind adding non-strangers whom I have decided not to be friends with in real life. I ignore them by clicking "ignore" on their friend requests. Some may keep trying. If they try more than two times with nice cute messages, I might accept it and carry on my life with as little Facebook activity as usual.

This man F, who I ignored twice, facebooked me and clearly told me his identity. Of course, man, I know who you are, which is the reason why I ignore you.
I was the best student in my class. Best, defined by Taiwanese education systems and belief, is getting the best grade in every subject, excluding art, music, housekeeping, or physical education. F was one of the worst students. He hated me for unknown reasons. I believed that he hated me. Now I do not know. Perhaps he thought it was real fun to curse to my face every time passing by me. He was the first person who ever threw the extremely offensive three-word phrase to me, more than several times. The phrase literately means fuck your mother. I was 12. I put up with his face and voice for three years. I do not need a friend like him.
To be fair, I admit a possibility that he may have changed or he may not remember what he had done to me. He was just an immature boy. But I still do not want to add him as my friend even though everyone knows that "friends" on Facebook belong to a different category of friendships.

Last Friday, I did something I should have not done in a milonga.
A beginner dancer, whom I tried not to dance with in a milonga, asked me to dance. I tried not to dance with him because I had danced with him in a practica and I knew that his bad habit was a no-no-no for me to choose a partner in a milonga. He has not learned how to lead with intension or with his upper body. He pushed me around with his arms forcefully. He has not learned how to listen to the follower or wait for me to finish a move. Very uncomfortable. He and I had chatted before, and it was not easy to reject a "friend". Here, friends are defined in yet another different way.
After a song, I hinted him that his arms were a bit stiff, and I encouraged him to loosen them up.
After the second song, I could not smile anymore. Sometimes, a leader like him could still keep a sense of connection through the pushing arms, but I felt nothing but being pushed.
I missed the moment to say "thank you". (Saying "thank you" to your partner on a dance floor means that I no longer want to dance with you tonight or for the next several tandas.) Therefore, I stayed until the end of the tanda. He was happy and not ready to say "thank you" to me. I couldn't help but asked if he was a ball-room dancer. He nodded. I said "You should loose the frame. It's too firm," he looked unhappy, and I continued "thank you."
He will not ask me to dance ever again. I know. I just threw away a potential good dancer if he will be one in a year.
He and I are not "friends" anymore. I guess.

Why do people have to befriend with their classmates?
I find it hard to do. I did not choose my classmates. They simply went to the same class as I did. If it is fine that they and I did not like each other back then, it should be totally fine that I decide not to get them involved in my current life in any form.
What if I offend them on Facebook? I honestly do not care that much. I would say "get a life" or "I bet you are not so needy that you have to have me as your Facebook friend."
Or should I be mean and click "reject" instead of "ignore"? For some people, getting a rejection feels better than being ignored. I do not know F well enough to know which one he prefers.

Why do people have to get comments so personally on a dance floor?
The fact that I dance poorly or wonderfully has nothing to do with other aspects of me. If I can listen to your opinions about my dance talents, it should be totally okay that I donate my two cents to you too.
But I know that I disobeyed the no-criticizing-your-partner-in-milonga rule. Even worse, this person happened to take my comments personally. His immediate facial expression made me feel bad.
Tango already invented a polite way to leave the floor by saying "thank you", but I still do not know how to avoid an invitation nicely or how to give verbal feedback nicely, nicely enough to leave feelings unhurt.
Eric suggested that I could have just stopped moving when being pushed. I replied "that's not nice."

I need something to click:
"Awesome"
"Good"
"Not bad"
"No comment"
"Helpless"
"Please disappear"

I wish the last option was offered on Facebook.