MacOS – How to kill a process by its name

This post suggest using monit.
As you have given no details as to the nature of the process, I can't advise whether their suggested config would be of any use for you.

killall kills by process name (which is definitively not 77439 and most probably also not Mathematica). You can use kill 77439 or (if this fails) kill -9 77439 instead (but if the process is really stuck, only a reboot will solve the problem).

Also, due the the way sending/processing of signals (like kill -9) works in Unix/OS X, there are situations where a process will be unkillable. One typical example for this is if a process gets stuck while accessing some external device and never actually gets control back. In such situations a reboot is the only way to get rid of the process.