Within the Case Options in Settings you have a section called Task Defaults:
In here you can decide whether or not you want Cases to be able to be completed with/without all Tasks attached to it also needing to be completed.
If you switch on the toggle, you'll allow Cases to be marked as complete, even if they have incomplete Tasks.
Remember to hit Save Changes when you're done to apply the settings to the firm's site - this will apply to every employee in the firm.
This will default to being off - so Cases won't be able to be completed until all Tasks within them are also completed.