Is there any way to disable the default behavior of the Insert key?

Go to Keyboard Shortcuts in System settings, then press Add button and insert a Name of your choice, say "DisableIns", and true as a harmless command then press Apply button.

Now, in the Custom Shortcuts section you will find an entry

DisableIns | Disabled

Click on Disabled, then perform your custom shortcut Ins. Press close, you're done.

I'm not sure it is the "right" way, but in my tests I could disable the behaviour of Insert using the following command:

$ xmodmap -e "keycode 118 ="

If this works for you, you could put the assignment in a file ~/.Xmodmap, i.e.

keycode 118 =

which should be loaded automatically at login. See questions here and here for more information about local xmodmap configuration.