I have found a solution to hide the join button, which works! This is taken from https://www.squarestylist.com/blog/workarounds
HIDE THE JOIN BUTTON
If you're going to use the Squarespace Member Areas, it's best to set up a free membership space so that you won't have to use its native checkout feature. Still, we'd have to work on safeguarding your premium content by adjusting the settings.
Whenever a person clicks on your member area without an account, they'll be redirected to an Access Denied screen, where they'd be prompted to join if they haven't yet. You'd have to hide this default member sign-up block using CSS and JavaScript so they can't bypass the payment gateway.
All you have to do is add the snippet below to Settings> Code Injection > Footer.
<!--Squarestylist Snippet to Hide Join Button-->
<script>
var sqelem= document.querySelector('#sqs-member-access-page-root .sqs-editable-button');
sqelem.remove();
</script>
<!--END SQSTYLIST JOIN BUTTON-->
Also, don't forget add this code snippet to Custom CSS. Press Cmd+Ctrl + ↓ to immediately get to the very end of your panel.
#sqs-member-access-page-root .sqs-editable-button {
display:none;
}