does anybody know how to turn off form actions please?

I have 2 types of users (members and non-members) using the same form action. This form has 4 buttons in total.

If the user is a member they have full access to all buttons.

If the user is a non-member they can only use 2 of the 4 buttons.

So, at the moment, when they are non-members and click on one of the members buttons, an OnClick event handler is added to the input type = "submit", which generates a new window with a document explaining to the user that they need to register.

There is no problem generating this window.

The problem is that the form specified in the form action still executes and the members page is still displayed.

So is it possible to turn off the form action in this instance so that the main page isn't refreshed/executed, or do I have to create different form actions for different users.

Surely there must be a way to prevent the form action from being triggered?

I realise I could simply not display the button for non-members, but I'd prefer to do so, as it allows them to see the features that members receive.

Thank you, if you can help,