Sorry there is no file called robot.txt it is robots.txt The Robot Exclusion Standard, also known as the Robots Exclusion Protocol or robots.txt protocol, is a convention to advising cooperating web crawlers and other web robots about accessing all or part of a website which is otherwise publicly viewable. Robots are often used by search engines to categorize and archive web sites, or by webmasters to proofread source code. The standard is different from, but can be used in conjunction with, Sitemaps, a robot inclusion standard for websites.
Please be aware that robots.txt can not protect your site against annoying spamming bots - they simply ignore robots.txt You can use Registration with Admin approval and / or good captcha in this not easy fight.
Robots.txt file is used to put on the link that you don't want the Google Spiders to crawl, it tells the Google Spiders not to crawl the specific page or link. I don't think that it can help you in removing the SPAM links ???? It's just there to direct the crawlers which page to crawl or not. For fighting with spam Google has provided you the Google Disavow tool.