In recent versions of the process list, there is no way to kill several processes matching a search string. With earlier versions you could do it as follows:
 1. Enter search string in the filter bar,
 2. Press Ctrl+A to select all filtered processes,
 3. Press Delete and confirm termination of the selected processes.

Now Ctrl+A does not select processes, it always changes focus to the filter bar.

Reproducible: Always