Is there a way to block compete.com or alexa from getting the traffic on my website? For competitive reasons, I don't want anyone knowing that info. I don't have advertising, just a paid-only site....would blocking compete.com or alexa in anyway affect my search engine rankings?
Is there a way to block compete.com or alexa from getting the traffic on my website
As far as I know, Alexa and Compete.com both use user-installed toolbars that analyze what sites the user visits (similar to how TV Ratings are done; see e.g. the Wikipedia article on the Alexa Toolbar). They do not fetch your traffic numbers as such; they estimate their own.
To mess with their counting process, you would have to block users with the Alexa Toolbar installed. While that may be possible, it's foolish in the extreme: You would be locking out innocent users, and decrease your Alexa ratings, which wouldn't affect them in the slightest.
Yes you can do it easily and automatically without having to contact them.
For Alexa, you can do it using robot.txt
User-agent: ia_archiver
Disallow: /
The toolbar plugin is used to monitor traffic.
I don't have details for the others, but I've just found some information about robot.txt for Compete, Quantcast and Alexa: look a the end of this article for practical details.
Please note that by adding that in your robot.txt it will also force them to remove all data they gathered retroactively.
This is useful for websites like archive.org (linked to Alexa, in fact ia in ia_archiver stands for Internet Archive). They explain the process here: http://www.archive.org/about/exclude.php
Please note that in most countries, being able to remove your data from databases is a legal right.