When you come face to face with your Ex with their new other half, How do you react ?

No differently when meeting another for the first time "unless" they give cause to address an issue. Which did happen on a couple of occasions;
One of which was an intoxicated moron as they sat together on the stoop in a drunken state who kept calling to my then 4yr. old son standing by my side to come sit on his lap having never met him or I before. I simply informed that just because he was fornicating his mother that was no cause for him to become an acceptable role model or even have reason to speak with him without my permission.
On a separate occasion with a different idiot, (we still lived in the same neighborhood), when my son was 5 we approached his mother because she had neglected to keep any of her visitations for some time and he saw her standing about a block away and wanted to say hi and possibly get a hug. I drove over to her and her friend and said good morning, how are you doing Iris? She ignored me, after looking over and continued to sip her coffee & talk with her friend. I politely said excuse me Iris your son is in the car with me, already knowing that she had seen him as he was by the passenger back window in his car seat facing her. She then stated yeah I know I just don't want to talk with either one of you rite now which almost brought our son to tears. Then I said; I understand it's just that he has been missing you and simply wanted to say hi and let you that he loves you. She simply looked around, said something to her friend and stared into space. I said; Iris can you please at least acknowledge your son and possibly we could talk about you spending time with him later if that is ok? She looked at house behind her, sipped her coffee and said; well I'm busy rite now! I was getting upset at this point... Then the moron spoke up and began to threaten me, stating something like; I don't care who f''k you are get the f''k out of her before I kick your stupid ***! At which point he soon realized that he had made a mistake. I got out of the car and walked towards them. Looked him in the eyes and said; Excuse me mother-f'er! Who do you think your talking to? She is the mother of our child. I am your worst nightmare, I been polite, understanding and patient and I did not ask you to speak one word you stupid douche bag, nor did I make any request to hear your pathetic opinion. Do you now understand me? Other wise I will rip out your throat and **** in lungs after beating some much needed sense into you! Is that clear? I will explain this situation to you once! She is nothing more then a *** receptacle and you are just the next donator in line. This has nothing to do with you, this is our child so unless you want to get your *** beaten into the ground here & now keep your mouth shut and get the f''k out of here. He left. Many may say there were better ways to handle it, I know. However he learned not to say what he couldn't back-up, our son received his much deserved hug & kiss and to talk with his mom for a mi

Tell her my friends and I want our love junk back.

I'd be nice. haven't seen her since she broke up with me over the phone. now she is married with one kid.

It depends on the circumstances. If it was a mutual break-up I smile. If I broke up with them I what to see their reaction first. If they broke up with me I act is if I'm ok because it wasn't to be.