This problem annoyed me too. (I like my custom hotkeys.)
I found this setting buried way deep in the system dconf settings under /org/gnome/shell/keybindings
on my system.
You'll need to install dconf-editor
sudo apt-get install dconf-editor
Then you'll need to search for "calendar" until you find the related setting.
(Note: I'm using Ubuntu 17.10 / gnome-shell window manager. This will likely be different under your system, but it should exist somewhere.)
(If you're using gnome-shell window manager: look under /org/gnome/shell/keybindings
for key toggle-message-tray
.
Here's what it looked like on my system.)
To change the default hotkey settings:
- Turn off the "Use default value" setting.
- Set "Custom value" to "". (That's two empty quotes)
- Now you should be able to use your custom keybindings.
Good luck, please post back here if this worked (or if you found the setting in 16.04)