There can be times where it will make sense to disable the campaign from showing on either desktop or mobile.
This could be for a number of reasons:
You do not want to show a campaign on one of the two devices because the campaign does not fit the target audience on that device
You want to have different conditions for your mobile and your desktop campaigns
You want a different integration on your mobile and desktop campaign
You might have other reasons as well, and that is why you will always be able to disable one of the devices.
Just go to the editor and hover your mouse over the device you would like to disable and choose either 'Disable desktop' or 'Disable mobile' respectively.
Here's how to do it:
Hover your mouse over Desktop or Mobile, then disable