I had exactly the same problem.
Regarding the manual deinstallation, Google came up with
this.
But before manually deleting it, I realized the root cause of the problem:
The plugin files are named
kunenasearch.* - but the plugin is initially configured with filename
kunena. As soon as you either change the plugin config to kunenasearch or rename the files to kunena.* the plugin becomes active.
Renaming is the better option because line 52 of kunenasearch.php looks for kunena as well:
$plugin = JPluginHelper::getPlugin ( 'search', 'kunena' );
Or, even better: try the file from SVN as mentioned
in this post