A shared worker can itself be permanently terminated by calling self.close() inside its JS. You can also send a message from the parent script to kill the worker:

// script.js

const awesomeworker = new SharedWorker('myworker.js');
awesomeworker.port.start();

awesomeworker.port.postMessage({type: 'cmd', action: 'die'});

We simply sent a message from our main script to our shared worker and passed the message that the shared worker should be terminated permanently.

The worker file looks like:

// myworker.js

addEventListener('connect', e => {
    const port = e.ports[0];
    port.start();
    port.addEventListener('message', event => {
        if(event.data.type == 'cmd' && event.data.action == 'die') {
            self.close(); // terminates worker
        }
    });
});

After verifying that the main script really wants to terminate the worker for all instances, the worker calls the close method on itself, which terminates it.