How to prevent users from registering without completing payment process

I'm starting to see people creating new accounts, but skipping the payment process, and still getting sent their login and password info. I've got Buddypress and membership working on a multisite. The process does work, but I don't want people in the system, if they are not paying.

Details: So, they click register, fill out the profile info, click submit, and it brings them to the paypal button to pay. At this point, they are stopping. But, Buddypress is still sending their info to them.
I realize I can limit their access, using membership. But, this doesn't seem right to me, and we're losing revenue.

Any advice?