By the way, ZD, I’m curious – where did you see this issur of eating pizza on Motzaei Shabbos?
Rabbi Dovid Meisels, in his sefer “Shabbos Secrets” says on page 126:
It is appropriate to urge people not to deviate from this ancient minhag. One should be cautioned against treating this meal with contempt, by eating non-Jewish dishes such as pizza on motza’ei Shabbos — even if the pizza has a reliable hechsher certification. The melava malka meal should have the character of a Shabbos meal
You can find it online by using Amazon’s search feature.
The Wolf