Disable KDE Chat application and system tray icon

If chat is starting at login, you can open the settings window from the contacts list, go to the "General" section, and uncheck "Restore last presence on login".

That doesn't disable the instant messaging icon in the panel (it just shows "offline" when it's not running). To get rid of the icon, right-click an empty space in the system tray (e.g. between icons) and choose "System Tray Settings". Go to the "Display" section and uncheck "Instant Messaging Presence".

It might be a little late but I just found out an easier method, in case anyone ever wonders in the future.

Go to "System Settings -> Startup and Shutdown -> Background Services". In the lower box which is labeled "Startup Services" you will be able to check and uncheck services in the "Use" column. Uncheck "Accounts" if you want to disable the Instant Messenger on startup which will also disable the tray icon. Log out and in again to see the change.

By the way, I am using Kubuntu 15.10.

You should uninstall kde-telepathy-desktop-applets package:

sudo apt-get remove kde-telepathy-desktop-applets

It should not get installed normally on upgrades.

Then re-logout into your KDE session. To apply changes to your taskbar immediately without re-logout restart plasmashell process:

pkill plasmashell; plasmashell &

Still if you want to make sure this package won't get installed (I've seen it installed automatically when upgrading 17.10 to 18.04) eventually you can blacklist it (see https://askubuntu.com/a/249447 for details) by creating /etc/apt/preferences.d/kde-telepathy-desktop-applets_blacklist file with the following content:

# Prevent this plasmoid from being installed.
# See https://askubuntu.com/a/995094 for details.
Package: kde-telepathy-desktop-applets
Pin: release *
Pin-Priority: -1