Its very interesting, You can block certain keywords in robots.txt so that bots will not index such urls which is having that particular keyword on the url. This approach is good for sites like user driven , classifieds . etc.
To Block a keyword just put the following Line in your robot file
Disallow: *xnxxx* Disallow: *tupe8* Disallow: *petardas* Disallow: *nxnn* Disallow: *kamapischi*
In red i specified the keywords.
Note: You have to be vary careful while playing with your robot.txt file