You want to find all the processes of, for example, visual studio code, and kill them all for the command line in linux.
To find all the processes ids for a particular app you can use the following:
Find all the processes for a particular app
For example to find all the processes for visual studio code (process name is code, you would type
Kill all the processes for that particular app
If you want to kill all those processes, just prepend kill and use the ids as arguments