Install openssh-server on Ubuntu and PuTTY on Windows. Then you can get a shell on your Ubuntu machine from the Windows one.
Once logged into your Ubuntu machine remotely, you can use kill or pkill or killall as you prefer. I find pkill simplest if you just want to kill based on the process's name. For example: pkill firefox
If, however, you wanted to kill a specific process whose PID (process ID) you know (possibly from looking through the output of ps -ef), you can use kill 1234
You can combine kill signals with these to say how they should be killed. The default is 15, or SIGTERM (term = terminate), which is a polite request to please close now. If a process doesn't respond to that, 9 (SIGKILL) is popular. See man 7 signal for more information about signals. To use this with kill or pkill you'd simply add -9 for example: kill -9 firefox