Disabling Mail and other unwanted programs?

What is there to disable? Just don't use it. You can use whatever whatever programs you want.

 

Just drag and drop the application in the trash bin if you aren't interested in using it.

 

Be aware that the Mac OS is going to try to prevent you from deleting some apps.

That's not really a problem.
If you don't wish to use them, don't -- it's that simple.

There are a number of alternative apps to use in place of Mail.
One that I use is Thunderbird...

 

The system will prevent you from deleting certain apps. Mail is one of those. You can force delete any app by logging on as a "Root" user but that is dangerous and can cause other problems. The correct answer has already been given. If you don't like an Apple installed app, then just don't use it. This isn't Windows.