Prevent user from unselecting option in selectizeInput

The default behavior for selectizeInput allows the user to click on a choice and delete it by hitting the Backspace or Delete key.

Is there any way of disabling that behavior?.

I have an app that allows users to add and remove groupings from the selectizeInput via ‘add’ and ‘remove’ action buttons. I don’t want users to remove choices with the selectizeInput widget but rather by using the ‘remove’ action button.