Disabling a content type is easy!
1. Go go Settings > Content Types
2. Hit the 'Gear' icon and a window will open up. Check the 'Disable Content Type' box - the content type will now be disabled.
3. Hit 'Save' when done.
This content type is now disabled.
You can also filter by showing only the active content types by toggling the Show/Hide Disabled button on the top right hand side of the screen: