Answer: Rambam (Shabbos 3:11) writes that one mustn’t put a ladle into a pot while it is on the flame on Shabbos as doing so stirs the food which aids the cooking. The Maggid Mishna explains that while it may not properly stir the food, chazal were concerned that one may come to stir it properly. Thus, the Shulchan Aruch (OC 252:1; 318:18) writes that one mustn’t stir food or remove any from a pot on the stove while it is still cooking.
The Mishna Berura (318:113) writes that while the Shulchan Aruch allows removing fully cooked food while it is still on the flame, the Elya Rabba writes that one should remove it from the flame first in deference of the minority opinion (Kol Bo quoted in Shaar Hatziyun 318:136) which maintains that stirring fully cooked food is assur mideoraisa (see Biur Halacha 318).
The Chazon Ish (OC 37:15) and R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 5:127:6), however, write that if the food was directly on the flame and so one wouldn’t be able to remove the pot and replace it, then they can follow the Shulchan Aruch and remove the food from the pot providing it was fully cooked (See Shemiras Shabbos Kehilchasa 1:32).
Following this, R’ Shmuel Wosner (Shevet Halevi 10:11:2) writes that if one had a pot that was too heavy to move off the flame, one could take from it directly, though one should be careful not to stir it.
Likewise, R’ Benzion Abba Shaul (Ohr Letzion 2:30:15) writes that if one needs to leave the pot on the flame for mitzva purposes, such as for seuda shelishis, late guests or children who wanted to eat later, then one can take food out of the pot while it is still on the flame.
R’ Moshe Feinstein (Igros Moshe OC 4:74 Bishul 9), however, disagrees, writing that even if the pot was heavy it must be removed from the flame before taking any. If the pot was on a blech or hotplate, however, then one may take from the pot, providing that the flame isn’t hot enough to have cooked it or warmed it up by itself (ibid. 11).
In conclusion, one may take some soup from a heavy pot if it is too difficult to remove, though one should be careful not to stir it. When possible, one should remove the pot.