How to, using PHP scripting or WordPress overwrite all of the posts (permamently, so it takes effect on database) with some static text such as "Content removed."?
Do not run this on a live site until you're tested it on a backup or local installation and confirmed valid results, i won't take any resposibility for lost data, should that occur.
To run from PhpMyAdmin
To use inside a plugin or your functions file (run once)
Another option, if you need further control of the query or the replacement function, is to use more of the WP APIs, like this:
Bear in mind that this is much more resource-intensive than the SQL queries t31os proposed, so use with caution if you're modifying a large number of posts. See also: How to apply content filter permanently?