The most likely classification for a single-celled organism with a cell wall but lacking a nucleus is Bacteria.

Here's why:

* Cell Wall: Both bacteria and plants have cell walls, but plants also have a nucleus.

* Lack of Nucleus: This characteristic is a defining feature of prokaryotes, which includes bacteria and archaea.

* Hot Thermal Vent: Bacteria are known to thrive in extreme environments, including hot thermal vents.

While archaea also lack a nucleus and are known to live in extreme environments, bacteria are more commonly found in these habitats.