Uninstalling Joomla is trivially easy.
Recursively delete all files in the Joomla docroot: rm -rf /path/to/JoomlaDocRoot. Surely you remember where you installed it, but if you’ve forgotten, this would probably be your Apache docroot or a subdirectory of it. On a local installation look in /var/www/html/, and on a remote web host check ~/public_html/.
Now drop the database associated with Joomla. phpMyAdmin is a good tool for this.
You might want to inspect the database before deleting it to make sure there isn’t any data from applications other than Joomla mixed up in there. Some cheap web hosts economize on databases, placing all your applications’ data into one big database rather than following the best practice of a separate database per application.