On Options chapter, in the page you linked, you can see the backdrop
option. Passing this option with value 'static'
will prevent closing the modal.
As @PedroVagner pointed on comments, you also can pass {keyboard: false}
to prevent closing the modal by pressing Esc.
If you opening the modal by js use:
$('#myModal').modal({backdrop: 'static', keyboard: false})
If you are using data attributes, use:
<button data-target="#myModal" data-toggle="modal" data-backdrop="static" data-keyboard="false">
Launch demo modal
</button>`