If you are using the code "as-is", the faster choice would be edit out all the htmls for second classes (class_master_XX.htm with XX = class code [0 human fighter, 1 warrior, 2 gladiator, and so on], removing the links for 3rd job).
If you know your AI, you could just remove the "class change function" (func_call 184680657 // func[ClassChange]) for 3rd job clauses (which enter by the ask=160&reply=-3), but you gotta remove whole "IF" blocks and not just one line.