It is not possible to hide or turn off the Join Us button and corresponding General Signup Form on your Hub. Volunteer interest doesn't end even if you don't have active opportunities available, so it's important to keep a channel open to potential volunteers.
While you can't close or remove the ability for volunteers to show interest in volunteering with your organization - you can change the permission settings and update communication so a volunteer knows what to expect. Here are a few things to consider:
Update Membership Settings and Welcome Message
Check on your Membership Settings. If you wish to manually review applications, make sure permissions are set 'To Review' for New Members instead of 'Auto-Accept'. This will place new membership applications into a pending state and allow you to review at a later date.
If your organization does not require the review of new members, it is definitely easier to have permissions set to 'Automatically Approve New Applications' for new members even during off-times. Then you can update the copy on your Welcome email to let volunteers know when your opportunities open again or next steps.
Update General Signup Form
I would add some copy to your hub and at the top of the General Signup form to let people know you are no longer accepting applications. Should any applications come in after that, they will be updated when anything new opens again. That way you don't lose any interested people but they have expectations set.
Update Thank You Screen
Another great place to update communication is the Thank You screen in the form edit view. Premium Plan customers can also customize the Thank You screen within specific opportunities. As it's the confirmation screen potential volunteers see when they submit their application, it's tends to have their full attention as they look towards next steps.