The event binding syntax in Angular maps between DOM events on a particular element and an Angular component method, as well as any optional parameters that need to be passed along:

<element (event)="method([params])"></element>

The ng-bootstrap modal defines a local alias to its own close method with the let-c="close" directive. This maps the close modal method in NgbModal to the local template property c, which can then be triggered like any other event. If you remove these local template input events, you will create a modal that can only be dismissed by clicking outside it. If you provide an optional backdrop parameter, you can define a modal that can only be closed by triggering the close event.

this.modalService.open(content, { backdrop: "static" });

Combining both of these customizations together would end up launching a modal you could never close, so be wary to leave at least one escape route available from the modal.