WP Admin- All User Tab

for some time I have wondered how in the world I wind up with thousands of users on the ALL USERS tab..currently I have over 1200 users..as subscribers
How do these folks get on there …

In the general settings tab I have the anyone can register tab unchecked…
I have an email subscription for for my RSS feed on the header..

I also have the comments box open only for post pages
but I never receive a comment and yet many of these
subscribers are leaving behind bogus email address and unrelated links to their sites…

How can I prevent this..obviously these people are spamming and looking for backlinks..
I always leave them unanswered and when I have the time I delete the whole bunch…

how does this happen and how can I prevent it.. ?