Need real help about two things

So i bought this wordpress based site from a guy (a blog actually) a couple of years ago and kept it on the backburner..now i am thinking of turning it into a full fledged website but i have a couple of problems.

a) A large number of users/black hatters have turned into subscribers even though the register mode in the front end is disabled. Consequently when i checked my site in google there were a large number of links to the sitename.com/author/name pages…i deleted all of them…is there any way that i could stop them (or others) from becoming “authors” again?

b) More importantly, he has a very big robots.txt file. Can someone help me structure it right? I mean i just want the search engines not to access the administration side of the site (i am assuming that was the reason why google indexed the authors pages) i just was the spiders to index the images and the posts that i publish. Can anyone please point me to the correct way to create such a robots.txt file?

Thanks