The Big One-
“jewishabortionist02” would be an inflamatory screen name, no two ways about it. It would be extremely difficult to be melamed zchus for such a name.
I have friends (some balei tshuva, some not) who believe abortion is OK.
I certainly let them know that that is lo kihalocha, but I do not cut off my association with them.
I have one friend who relies on an eiruv that I do not, and at my request he does not carry when he comes to us on Shabbos.
The bottom line is I don’t have all the answers as to how to deal with different people (I asked Rav Feivel Cohen a shaila how to approach the Shabbos guest).
The “abortion” question you posed is a lot more black-and-white than the “red clothing” question, and an aborted child is much more of a tragedy than the wearing of red clothing.
To answer “What would the appropriate response be in your view?” – the first time, I would reply that is was kneged halacha, and show where.
If the person kept insisting that they were right, then there are a few choices: 1) Continue to argue, and try to bring additional rayos. 2) Decide that continued discussion won’t change the person’s mind and end the topic. 3) Tell the person that they are a <>. 4) Etc.
Part of chanoch lenar al pi darko is taking the best approach – factoring in the kavono and hashkofo of the other person, and in public (such as these threads) how your statements will affect and be percieved by others.
There have been people here who identified themselves as baalei tshuva, geirim and yesomim. We also don’t know who may be “teetering” and turned off by a blast of what they perceive as harsh, intolerant rhetoric.
One other factor is that unlike spoken conversation, these threads are close to ksav asher lo yimachek. Once something is posted it can’t be untyped.
This is not the concise answer you probably requested, but this is my philosophy when answering others.