Sending programs to the Trash bin will usually keep them out of your hair. However, some programs bury themselves deep into your Mac's settings and Preferences folder. A simple delete won't completely rid your Mac of those.
To take them out, you're going to have to go deeper. You don't have to go it alone, though. You can nuke deeply-rooted programs with App Cleaner.
It will uproot any undead programs it can find. It can also monitor your Trash can for deleted programs and take away nasty files associated with them automatically. These saves time and necessary system resources on your computer.