Try browsing (with nemo) to
(where username is your user name).
Switch on "show hidden files" in nemo.
You should find a directory called 'Trash' (or similar name).
Delete it. This will also delete its sub-directories.
Your Trash icon should now show your bin is empty.
That Trash folder will be automatically recreated whenever it is needed in the future.
xkill - kill a client by its X resource
Don't do this!
Try installing htop - a fancy version of top,
or if you wanted to (for example) kill firefox, try something like
ps aux | grep firefox
This would display several lines, the last of which is your command entry, so you ignore that one.
jeremy@hector ~ $ ps aux | grep -i firefox
jeremy 8178 5.7 5.1 1364396 413368 ? Sl 16:17 23:56 /opt/firefox/firefox
jeremy 10328 0.0 0.0 12724 2108 pts/0 S+ 23:16 0:00 grep --colour=auto -i firefox
That 8178 is the PID, so you enter