How can I go about creating an iPhone-only app?

You must indicate it in the options of your xcode project, and also in the info, if you upload it without the info, apple people do tests on ipads and tell you that it does not look good, so the device capabilities are important from info.

Another important point is, the Capabilities of the Info can be changed as long as you have not released the app, if you have already released it you will not be able to change it and upload it because the Apple tell you that you cannot restrict a release if the previous one covered more devices.

Here is the project configuration:

And here is the Required devices Capabilities, in this option it has the armv7 and that value will be changed by telephony

To see the different options I attach the apple link.

Device Compatibility

Once you have the configuration right, it should remove the ipads option from itunes connect, obviously you have to upload the compiled file to itunes and select it to release or test.

When you create the project you can tell it for which device you want to do it.

Attached image.

You can also modify it in the project properties.

Adding to Shadros answer, you can change it once you have already created the project in your app properties.

Click on your project, and in the 'General' tab you will find a Deployment Info section.

Hope it has fit.