if you want to block other crawler except Google,Yahoo and bing crawler.so please you can use following syntax. User-agent: Googlebot Disallow: User-agent: Yahoo-slurp Disallow: User-agent: Msnbot Disallow: User-agent: * Disallow:/
robot.txt are not worked always but if you update them every time with changing security then its works best also some java text can make help batter
There are some search engine which is popular in exact coutry: lets say yandex is most popular in Russia. So if you dont want to be searchable in Russia block yandex bot, it will stop indexing your site, less non targeted users will come, you will save some hosting traffic as well.