Making a membership plan compulsory at registration

Hi all :)

Is there a way to make a membership plan compulsory at registration?
In other words, if one does not choose a plan they cannot register.

Thanks in advance ;)

P.S : and if you do not choose a plan (you're only a visitor) you can neither contact a seller (I've figured how to do that) or buy a product (when using the Shopping Cart plugin which I'm yet to buy).

Hello,

Actually, if you don't select a plan you can't go further in registration

Thanks. :)