Hello
A good first step to help keep your server free of cheaters would be to sign your server up to stream to pbbans.
Might want to check out the starters guide here.
http://www.pbbans.com/info-center-starterguide.html
Also might want to check out the Hub Streaming Guide here.
http://www.pbbans.com/info-center-hubguide.html
As for the black screenshots there is a number of different things that can cause that. Client server lag, Alt tabbing, Vista, Anti Aliasing can cause it. Also other things.