It's really tough to prevent determined spammers from flooding the front page with garbage...Many are spam-bots with multiple usernames all advertising the same junk....Illegal sports streaming etc. In recent weeks it's been a goldmine for them with soccer and other high profile events taking place.
The best course of action is to report them...This helps the Mods to delete them, but believe me, it's not confined to MR. It's annoying I know, but as posted above, it's a clear indication of a sites' popularity when spammers target in such a determined fashion. There are other less popular sites that remain relatively spam free....But they are also relatively member and topic free also. I expect with the forthcoming Olympic Games in London, we will probably see an increase....Wimbledon tennis is also a prime target as well. Report all you can and they will be taken care of.