You should be able to simply remove the domain from the DreamHost hosting system by going to Domains -> Manage Domains in the panel and clicking the small X under the Web Hosting column.
After a little while, the domain should throw an error when you try to browse to it.
Edit: Another option would be to create a .htaccess file with the line below and place it in the domain's web directory.
This will result in a 'Forbidden' error if you try to browse to the domain.
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