Unwanted request for registrations after new post

All,

I have installed a plugin (New User Approve) that manages my registrations so I can reject, hold or accept a new registration. And only registered users are able to read my blog part of my site.

Whenever I post a new blogpost to the protected area I get a 25 -50 new registrations from spam accounts ( most of them yahoo).

My questions are:
– What triggers this. Is my site affected somehow to tel it is active?
– should I reject these registrations ( and let them know they reached a valid WP site) or should I keep them on hold.
– For now I keep them on hold, but where could i delete these from the DB, as he amount is growing.

Thanks for any pointer or hints.

Karel