Which web server are you using?
, (and you have Admin access) open IIS Manager, right-click on your text file, select properties, then go to the HTTP Headers tab. From there, Enable Content Expiration and select Expire Immediately. This should cause an entry in the HTTP header that a properly-behaving cache should obey and not store the information. If that doesn't work, sometimes you can play around with Expire After (Set it to 0 or 1 minute) and Expire On (Set to a prior date) to get it to work. Some caches are nonobedient, and sometimes IIS does weird unexpected things.
, you can use the functionality of the Apache module mod_expires. It's included with the base Apache source, but I don't think it is compiled by default. You'll need to check to see if it was included during compilation. (If not, use APXS to add it (apxs -i -a -c mod_expires.c) then restart Apache) I don't think it is added by default.
With mod_expires in place, add something like this to httpd.conf:
For more info on mod_expires see http://httpd.apache.org/docs/mod/mod_expires.html
If you are using a different web server, I have no idea. :)