A robots txt is a text file uploaded to your website server to keep compliant search engine robots from indexing particular pages of your website. Create a robots txt file like this

User-agent: Search Engine Robot Name
Disallow: File or Directory

Ex.

User-agent: Googlebot
Disallow: /adcampaign1.html

or

User-agent: *
Disallow: /adcampaign1.html

The * is used to cover all compliant search engine robots.

Common Robot Names

Search engine robots follow links throughout the Internet to crawl through all the websites on the world wide web. Here's a list of top search engine robot names:

Google -----> googlebot

Ask Jeeves -----> Teoma

Alta Vista / Yahoo / All the Web / Inktomi -----> Slurp

Check Your Robots txt on Search Engine World's Robots.txt Validator.