New Post: restricting registration

Hi All,
I have a problem that is now occurring on a number of my client online stores. The sites are getting A LOT (100's each night) of new user registrations from dubious sources.

I want to be able to hide the site registration option so that the only time a user is shown it is when they are required to login to the shopping cart as part of the purchase.

Can someone please tell me if this is possible and if so, how do I set it up.

TIA
MartyNZ