How do I kill a process that won't die?

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.

On newer os, use launchctl to trigger a reboot of the apps / user space.

launchctl reboot apps
launchctl reboot logout
sudo launchctl reboot userspace

I would start with apps after saving your work, then progress to a logout and then the userspace reboot. You shouldn’t need to restart the entire OS unless things are really broken far past one stuck process. Stop as soon as the first one of the above commands gives you relief.

You may find an iPad or iPhone or another computer to ssh in makes it easier to quit things since if you run terminal app, it will exit after the first command.