When a discussion with a young earthist starts going round and round in circles, it is a good idea to see if you can figure out why, and whether continued discussion is a good use of your time.
One particular red flag is when they keep repeating the same falsehood over and over and over and over again, despite the fact that you’ve repeatedly corrected them on the matter. For example, I have repeatedly made the point that deep geological time is established on the basis of rules and principles that have nothing whatsoever to do with secularism, but that apply to Christians and atheists alike, and that apply whether you acknowledge miracles as a legitimate explanation for scientific observations or not. Yet when I see individuals to whom I have repeatedly made this point continuing to denounce deep geological time as “secular scientific interpretations,” without even showing a shred of concern as to what the rules I am referring to actually are, it becomes abundantly clear that they are not listening, they are only interested in shouting, and in all likelihood they are approaching the discussions in bad faith.
At such a point, it is no longer fruitful to continue the discussion with them, and so the best course of action is to just ignore them. Some such people act like this because they are trolling and get a kick out of seeing knee-jerk reactions to bad arguments, so the best course of action here is to deny them that particular satisfaction.
Internet forums present a bit of a challenge here however, because you aren’t responding just for their benefit but for the benefit of everyone else following the discussion. Nevertheless, there does come a point at which “don’t feed the trolls” becomes advice that we should all seriously consider following. Besides, it is often the case in such situations that the badness of their arguments speaks for itself.